youTube.com is an amazing site. Not only does it contain hilarious videos of cats, it also enables you to see children falling off bicycles. Before youTube I used to have to fling cats at children on bicycles, but no longer. It's win/Win/WIN! Part of its genius is the name - it starts with "you" so it makes everyone feels like it's relevant. The principle could be used to start up a bunch of other websites which I think could end up being just as big. Here's a partial list - feel free to register and develop them yourself - I just don't have the time to give them all that they deserve.
youThyroid.com - This site would be handy for people to upload pictures of their goitres or of their Mel Brooks googly eyes. They could trade stories about their pretibial mixoedema, and swap tips on how to use the internet effectively despite a severe tremor and a heart rate of 160.
youVolaemia.com - This site would be dedicated to enabling people to share hints, tips and tricks to do with their fluid balance. How to best assess your jugular venous pressure. The controversy over mucus membranes. Skin turgor and the elderly. The osmolality conspiracy.
youRopeanUnion.com - A fan site for European communalists of all stripes!
youClideanGeometry.com - Do your external angles sum to 360 degrees? This could be the site for you! Or perhaps that's just hyperbole.
youKaryote.com - Too long have plants and fungi been excluded from the internet. This site would serve as a portal for all organisms with nuclei.
youPhemism.com - This site would have a lot of the you-know-what, eh! Eh! Especially for you-know-who!
youLersNameIsPronouncedOilerNotYoulerSoThisJokeDoesntWork.com - A site for looking up the names of dead mathematicians and the mispronunciations thereof.