CNet has done an article on their Top 10 Internet Fads. I'm not sure if I agree with most of them, some of them don't seem to really be fads, but it's still interesting. Do you remember "All your Base" or The Dancing Baby or the Hamster Dance? Man, how embarrassing. Although I guess I'm not one to talk, this website alone seems to point out enough viral and popular sites and movies as it is.
A better resource might be Wikipedia's Internet Fads, which has hundreds of examples. It's a pretty huge list so it'll take a while to go through.
Link (via Waxy)