Duke Nukem Forever is a decade-long example due to Development Hell and Vaporware; it was "coming soon" for 14 years before finally being released in June 2011 (after suffering one last one month delay, a fact the developers readily lampshaded). It took transferring property of the game from 3DRealms to Gearbox to make it happen.
All Blizzard products have a release time of "Soon" (TM). Diablo III began development in 2001 shortly after the release of the Diablo II expansion pack. It would be eleven years before it would finally release. It wasn't officially announced until 2008, and then didn't hit beta until 2011, before finally releasing in 2012.
Meanwhile, the long-promised 1.13 patch for Diablo II took two years to come out after its previous patch, finally dropping in 2010. Then again, it's impressive that Blizzard continued to patch a game that was, at that point, nine years old.
After a 12-year wait, StarCraft II: Wings of Liberty was released, only to have one of the three races (Terran) come with a Single Player campaign. In order to play Zerg 1P, you will have to purchase a full-blown Mission Pack Sequel called Heart of the Swarm, which, was still in development two years after ; open beta was announced (not started, announced) in August 2012 and it finally released in March 2013. And Legacy of the Void, the Toss campaign, is still a gleam in Blizzard's eye. By contrast, Brood War, a "mere" Expansion Pack, came out the same year as the original Starcraft with all three races given fully playable single player campaigns, just like in the original. (On the other hand, while SC 1 gave you three races with 10 missions each, Wings of Liberty shipped with a Terran campaign 26 missions all on its own.)
Averted with World of Warcraft: Cataclysm when it was released prior to 2011 on December 7, 2010. It was an Obvious Beta compared to other expansions, but after Burning Crusade had to be pushed back to 2007 (Original release date was sometime in November 2006) and the game was playable, that they could launch it.
Averted with World of Warcraft: Mists of Pandaria only because they didn't give any release date at all up until two months prior to release. (Although lots of information had been released about development and there was an open beta, so it wasn't a big surprise.)
Valve, the developers of Half-Life, Team Fortress 2, and Left 4 Dead are particularly infamous for continued delays on their release dates - so much so that their fandom has coined the term "Valve Time" to refer to the manner in which they can single-handedly dilate time around themselves. This is probably mostly thanks to Valve's loose internal corporate structure, where no-one has a single, set-in-stone role to play in the company. They claim it is due to their attempts to produce high-quality games that are fun, and the developer commentary often mentions features that were not fun that had to be discarded, which certainly consumes time. Even Valve's own wiki pokes fun at it:
Averted with Left 4 Dead 2. Valve intentionally released the game on a fixed schedule (literally a year after the first game was released) to prove they can release on time. Fans to this day still argue whether or not this was a good thing, due to several issues the game brought up.
Valve Time kicked in for The Passing DLC, although not too extreme. It was announced to be released around March 2010 and it got delayed by a month. The Cold Stream DLC was a much bigger example of Valve Time; the beta was released in March of 2011 and several release date windows were posted and passed (end of summer, on Halloween, and end of the year) and the full version wasn't released until July of 2012. Even the Xbox 360 version wasn't released on time due to some update problems and would be released a week later. This left Xbox owners extremely upset due to all the waiting.
In some cases like the above, there's also "Reverse Valve Time".
Due to the (relatively) long time between the release of The Elder Scrolls V: Skyrim and the Mod API for said game, and the fact that the hold up seems to be due to problems with incorporating Steamworks support, its become a minor meme to say that Bethesda is running on Valve Time.
Black Mesa, the fan remake of Half-Life on the Source engine, released a trailer claiming the game would be released in 2009. This claim stood until early 2010 when they stopped giving any estimates on a release date until around when it was finally released in late 2012.
The September 2012 released wasn't even the entire game. The last four chapters (the Xen section) have yet to be completed.
Krin, the guy responsible for the Sonny games has no definite release date for Sonny 3. Members on the Armor Games forums frequently speculate when Sonny 3 will come out. For a time, people actually thought the guy was dead.
Daikatana: In 1997, an ad is released that promises that "John Romero's about to make you his bitch". At E3 1997, a Christmas release is promised and missed. Christmas 1998 was promised and missed. Eventually, Romero claims that "Come hell or high water, the game will be done on February 15, 1999." Well, it wasn't. (It finally was in April 14, 2000.)
Super Smash Bros. Brawl was announced in 2006 and set for a 2007 release. But because they already had one third-party character in the game (Snake), and fans demanded another third-party character to be added (Sonic the Hedgehog), the game had to be delayed until Feb. 2008. Then when February was near, the game was delayed until March because they wanted to work out some bugs in the online system (although the online was still very buggy even in the finished product), and so the game finally saw a release in March.
Kid Icarus: Uprising also went through several delays too. It was initially announced in 2010 as a launch title for the Nintendo 3DS, but when the 3DS's launch neared, it was delayed until August of that year. August came around, and the game was delayed until November or December. Then it was delayed until early 2012, which was when it was finally released.
Super Smash Bros. for Nintendo 3DS and Wii U. The 3DS counterpart was going to be released in summer 2014, but will be released in October instead.
Gears Of War 3 was set for release in Spring 2011 but ended up being pushed back to September 2011. Officially it was to polish the game and work out balancing issues with the multiplayer (don't go there) but for most it seemed like a blatant attempt to get it released around the holiday season.
Working Designs was notorious for this. ADV Films delayed the Arc the Lad anime to come out the same time as the game; at that point they had already been late so often that the Genre Savvy could have predicted that wouldn't work. Magic Knight Rayearth also ended up being the last game for the Sega Saturn (outside of Japan, anyway) because of various delays.
Rayman Legends was announced in early 2012 as a Wii U launch title. Later in the year, when the WiiU was nearing launch, it was delayed until February 2013, because the game still wasn't finished yet. Then when February neared, the game was delayed until September, to coincide with the multiplatform release (a decision made because of ZombiU's sales not meeting expectations), even though the Wii U version was already 100% complete and ready to ship.
In 2006, Square Enix announced a game for PS3 called Final Fantasy Versus XIII with an elaborate trailer and a promise of "Coming soon", but production hadn't even started yet. In 2009, the first actual gameplay footage is shown. After this, very little is said about the game other than the occasional trailer and the promise that it still exists in spite of a lot of the staff being pulled away to work on other projects. The game would not go into full production until 2011, by which time the next set of consoles were already being talked about internally. At E3 2013, a full eight years after the initial announcement of Versus XIII, they confirmed that the game would eventually see release. As a multiplatform game called Final Fantasy XV, still with no release date.
Star Citizen was announced for a November 2014 release but has had the date pushed back to early 2015.
The Legend of Zelda: Twilight Princess was initially slated for June 2005 for the Gamecube release but ended up being a Wii launch title, on November 2006, 17 months later after having the release date pushed back no less than 5 times. In fact, the Zelda games are so often delayed that only two 3D games averted this: The Legend of Zelda: Majora's Mask (which had to reuse several elements from Ocarina of Time to be ready in only one year) and The Legend of Zelda: The Wind Waker (for which they had to rush development, resulting in two dungeons cut).
An entire game console was infamous for having this happen with most of its games. The Nintendo 64, due to how difficult it was to develop games for (thanks to both the cartridge unit and some rather odd hardware design choices - such as no dedicated sound chip), was notorious for its games being constantly delayed, and for long waits inbetween games. It wasn't uncommon for Playstation 1 and Sega Saturn games to be released a good year or two later on the system, and it was frequently criticized by both the press and gamers alike for its relative lack of games. The situation became so bad, in fact, that Nintendo frequently tried to justify it by saying that they believed in quality over quantity. Most of the games that weren't developed by Nintendo or Rare are considered to be So Okay, It's Average at best.
Scribblenauts Unlimited's European release was stated for February 2013. While the PC version was released that month, the Wii U/3DS versions were pushed to March, then April, then May, then September and finally were set in a Nintendo Direct for December 2013.
There was going to be an indie game for the Wii called Sadness, which would have been a survival horror game portrayed entirely in black-and-white. It was originally announced in 2006, but as the years passed, no further information was given, not even an actual gameplay trailer or screenshots of any kind, and the title was officially cancelled in 2010.
The Last Guardian was announced in 2007 or 2008, and still, practically no information has been given on it. Sony Computer Entertainment confirmed as of early 2013 that it is "still in development", but haven't given any further details.
Starbound was initially slated for release in summer of 2013, but missed its mark. The dev team then settled on a loose release date of "when it's done", but released the beta of the game in December 2013.
Dead State is having some issues with this. Mainly because a quarter of the Dev Team lives in Ukraine which is undergoing political upheaval
An inherent aspect of any crowdfunded game that got noticeably more than the base amount they asked for (such as the aforementioned Star Citizen, or Wasteland 2). The reason is, of course, that the original estimated release date is based on being able to just make the game, a larger budget means more stuff being put in the game, and some of that stuff will take more time to develop than the original schedule (you can only get so far with hiring more people).
Bayonetta 2 was said to be released in the summer of 2014 during February 2014's Nintendo Direct. Nintendo's E3 stream later that year stated that it will instead be released in the fall (September for Japan, October everywhere else).