(sic)
Once upon a time, there was a website in Geocities that somehow attracted public attention. Just like in Mahir's case, no one really knows why it happened - perharps because it was because some people thought it was funny.
On the page, there were lots and lots of dancing hamsters as animated GIFs. Those who had (mis)configured browsers could hear that they were dancing to the tune of modified song from Disney's Robin Hood movie.
Yes, like I said - no one knows why this was popular. Maybe, maybe some people found it funny.
The page was really titled "Hampster dance" (sic), and later it got a domain of its own.
Like all other good internet memes, this was somewhat messed around with. Look-alikes started to show up. Personally, I remember seeing a site called "Pikachu Dance".
I remember some people used the site as browser benchmark. (I use similiar gauge when rating homepages of peelos. If my processor load gets higher than when browsing Hampsterdance-like page, I often notify the page authors...)
(TODO: Factchecking and perhaps more history. I'll try to find the site after I've written this, because I'm writing this from NS4 and it might crash when I get to the page =)
See also: the next hampsterdance, Hamster Dance
Okay, so there's another node called Hamster Dance. Don't blame me, blame the inadequacy of E2 searches and considerable amount of too strong coffee... It isn't my fault that the original webpage's author couldn't spell worth a damn, and it's even less my fault that other people are smarter than him =)