Duke Nukem Forever is the epitomeof this trope, as 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, nearly two years after Wings of Liberty's release, is still in development; open beta was announced (not started, announced) in August 2012. 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.
In fairness, 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, which has been praised for its thorough and creative design.
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.
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.
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.
X Rebirth was initially announced for a fourth quarter 2011 release, but is now expected in mid-2013 or thereabouts. The devs and beta testers are barred from talking freely about it thanks to non-disclosure agreements imposed by the publisher, meaning some fans are half-convinced it's become vaporware based on the lack of information. CBJ, the one dev who visits the forum on a regular basis, consistently denies this to be the case. Hype Backlash is starting to set in, especially when one ill-advised poster compared the news blackout to a Nazi regime. Yeah, the moderators loved that... (Factor in that Egosoft is based in Germany.)
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 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, because Ubisoft wanted to make things fair for the other gamers and port it to other consoles too.
Final Fantasy XV was first shown in 2006 as Versus XIII for PS3, only to suffer 7 years of schedule slip till it became XV for PS 4.
