4chan
4chan is an English-language image board that was created in 2003 by Christopher "moot" Poole as an American version of the Japanese Futaba Channel (2chan). He translated the source code of the Japanese image board to English, then started up what has been the most famous (and infamous) Engish language image board of all time.
4chan was originally created for the discussion of anime, but has grown into a collection of image boards dedicated to all sorts of topics. Posting is anonymous, though nicknames can be used.
However, it isn't just images 4chan is known for. 4chan has been the birthplace of lots of internet trolling, contributed a lot to the culture and formation of Encyclopedia Dramatica (both its former and current incarnations), started a ton of internet memes, and has been the center of a ridiculous amount of publicity, both good and bad.
As a side note, this very page was created since TV Tropes.org refuses to have a page for it, as documented in this thread.
- GIFT - There is a very good reason this site is called the "asshole of the internet".
- Memetic Mutation - Tons of memes have been born due to 4chan.
- Rules of the Internet - Created thanks to 4chan.
- Rule 34 - The amount of this you can find on 4chan is mindboggling.
- Troll - Tons of these can be found on 4chan too.