4chan is a huge image-based message board based on the Japanese site 2chan (for 2channel). 4chan is crazy about all things Japanese, but especially anime, and most especially hentai.
4chan is often attributed as the source of new internet fads or memes. This reputation is no doubt exaggerated, but still, many of the phrases and pictures that become popular on Board 8, LUElinks, and other boards do originate at 4chan (though a site like Something Awful probably has as large an impact).
The site became a major source of controversy during the '07 Character Battle when it was rumored that the first round success of many Pokemon (especially Mudkip) was due to massive 4chan vote stuffing. Eventually we learned that, although some rallying did occur in the early stages of a few matches, 4chan's impact on the end results were always relatively minor; Pokemon are just a legitimately strong force on GameFAQs, at least in a 4-way environment.
Nevertheless, ever since the Summer of 2007 it has become a trend to blame 4chan whenever anything unexpected happens in a GameFAQs poll.
The site was once again the source of major controversy during the Gamespot 2009 Contest when they rallied for Bub & Bob. Because of the lower vote count in that contest, 4chan was able to make a massive impact on the contest, leading the duo to victory over Master Chief and Sonic the Hedgehog. When it came to the match against Samus Aran, there was evidence of massive vote stuffing, and GameSpot ended up removing around 16,000 fraudulent votes from the duo and handing the match to Samus. 4chan retaliated by launching a DDOS attack on GameSpot, which was successful in that it brought the entire site down for 10 minutes, though unsuccessful in that they got bored after 15 minutes and went on with their lives.