The Agony Booth started out with a few fairly long, infrequently published recap-plus-snark treatments of fairly obscure, terrible movies, with a handful of screencaps per page, written mostly written by the site creator. As the site gained a cult following it developed an expanding stable of writers and a broader portfolio of subjects, covering everything from notoriously bad B-movies to flopped blockbusters to offbeat television series like Mister T, with an increasing emphasis on more involved, humorously detailed critiques of every scene, multi-author recaps, and clever captions for the more frequent screencaps. Then, seeking a more mainstream audience, around 2010 it evolved again, deemphasizing the long text recaps in favor of video recaps and essays, usually in the form of ongoing, named segments from recurring contributors.
Maddox's early articles were usually much shorter than his current ones. He also wrote about random things like making sandwiches and replacing the moon with a giant robotic monkey head, and he regularly complained about his older brother.
Early articles from the SCP Foundation (note: an SCP's number has nothing to do with how old it is, although the 1000-1999s are generally newer, as with the 2000-2999s. You will never find an "old" article in either of those groups.) are frequently different in style and tone from the currently accepted 'standards', as they were written before said standards were generally agreed upon. Compare SCP-173's article, for example, with almost any of the others'. The general rule of the wiki is that you don't edit another author's work unless the article is in danger of being deleted, and since many of the early authors are no longer on the site and the surviving SCP's are fairly popular, that means they're unlikely to be altered any time soon.
Looking at early drafts of Wikipedia articles, it's hard to believe it's the same website. Articles were far shorter, and they didn't even have links to other articles. There were no categories or images. Also, they initially used CamelCase to create wikilinks instead of brackets.
Also, stricter policies regarding "fair use" reduced usage of copyrighted material. And the standards of article quality are now much higher, something frequently brought up in Featured Article Reviews (where articles which were considered among the best are analyzed to see if they aren't worthy any more). It's easier to illustrate when an FA gets demoted and re-promoted to show how things changed.
Diva Dirt only had two writers when it first started out and was very biased in its writing. Both Melanie and Erin would Accentuate the Negative a lot in their articles and headlines as well as mostly focusing on WWE. When more writers came to the site, it expanded to include independent wrestling and eventually MMA too. These days the personal opinions of the writers are confined to the columns and are much more formal and neutral.
Gaia Online was originally a very little known Animesque roleplaying community with a list of links to anime fan sites and such, and the site's earliest graphics were inspired heavily by Ragnarok Online. It also used to have a dash in the url, go-gaia.com, but the dash, the RO-inspired graphics, the link list, and the emphasis on a roleplaying community were all dropped as the site grew. Today, it's more of a social networking/social gaming hub, although roleplayers are still welcome, by all means.
One of their most prominent Cash Shop item types, Evolving Items, also looked very different when they first arrived. Evolving Items are items that start with a few poses, the first "generation", which is gradually replaced by later generation poses according to an update schedule, until the item reaches its conclusion, where all of the previous generation poses the item comes with are unlocked as well as the final generation poses. In the beginning, these Evolving Items chronicled the growth, or evolution, of a creature or plant of some kind, and didn't really have much of a story beyond that, aside from the growing plot surrounding the hosts of the weekly Evolving Item Report. Things changed in a very big way at some point, and the Evolving Items now almost always have a backstory and tell some sort of story themselves.
On the subject of said report hosts, there had initially been a Running Gag in which every week, their avatar would be changed to reflect whatever they'd been doing. After Timmy was turned into an Evolving Item himself, this was dropped and Dr. Singh's avatar just stays the same.
Neopets has evolved considerably since starting out in 1999. In addition to growing Lighter and Softer over the years (it was originally targeted at college kids), it has also seen considerable Art Evolution that has completely changed the design of more than a few species. (One of the most notable being Bruce, which was originally a picture of Bruce Forsyth before changing to a penguin-like creature.)
Facebook. In its earliest form, it was completely restricted to a handful of colleges, and intended only to be used for socializing on college campuses. It also didn't have status updates for the first five years of its existence, and even then all status updates were in the format of "(name) is _____" until a later update dropped the "is" to improve status update flexibility. Facebook didn't even add the "Like" button that has since become a part of its current identity until the year after that.
YouTube videos from the first few years end up looking odd, since the video makers often reference UI features that YouTube has since changed: you can no longer "five-star" videos since they changed to thumbs-up/thumbs-down in 2010, and the description has moved from the right to underneath (which means people pointing while saying "link in the description" are now pointing in the wrong direction).
Vat19 used to create and sell DVDs before becoming purveyors of curiously awesome products.
This Very Wiki used to list normal, YMMV, Flame Bait and Trivia examples all together on work pages until mid-2010. After that there was a period of a few weeks where both YMMV and Flame Bait examples were completely banned from work pages and could only be listed on trope pages, but it quickly became obvious that this wasn't going to work out and resulted in the current format of separate YMMV (and later Trivia) pages, with Flame Bait examples staying restricted to the trope pages (and sometimes not even being allowed there either). We used to have Troper Tales on this site too, but we prefer not to talk about that.
A lot of the early stories were simply one random line spoken by a customer (or the occasional "grab bag" consisting of several randomly-culled lines from assorted customers), instead of longer anecdotes.
For much of the site's early history, only select stories were sorted by theme (e.g., "Bigotry", "Awesome Customers", "Geeks Rule", etc.). Now, every story is.
Some of the earlier stories were copied wholesale from other sites, most often Rinkworks. Eventually they settled on only taking original, user-submitted content.
In the early days of the photoblog "Humans of New York", photographer Brandon Stanton would include editorial comments on his photos much more frequently, and he would customarily mark each photo with the date that it was taken. Many of the blog's early photos aren't even of people in New York, but are just casual posts about Brandon's daily grind as a photographer. Nowadays, the blog is probably best known for its candid street interviews, and Brandon practically never gives comments on his photos unless his subject doesn't offer a statement of their own.
Crunchyroll originally showed pirated fansubs of anime, before eventually going legit and becoming a source for official streaming.
Reddit didn't introduce subreddits until 2008, 3 years after the site's launch.
Twitter was originally conceived as an SMS-exclusive service, which explains the persistent tweet character limit of 140 characters, until eventually adding a web interface and eventually an API for developers to make third-party apps with. Likes/Favorites, retweets, #hashtags, and even tagging users with their @-usernames were not official features for the longest time. This is in contrast to modern Twitter, which has since added a slew of features that have made using Twitter exclusively via SMS less and less feasible, and there's even rumors of increasing the character limit, which may as well mean the outright end of SMS Twitter.