Mathworld is a mathematical encyclopedia on the Web. Up until now, I thought it was the only one. I was a bit annoyed at having to use Mathworld because it is owned by the Mathematica people and so, you never know when they won’t pull a Microsoft on you.
Didier (who I wrongly assumed to be from France initially) pointed out PlanetMath. The cool thing about PlanetMath is that the content is great and released under GPL. This means that they won’t pull a Microsoft on you! You can copy the content and redistribute it if you so wish. They can close their servers, but the data itself is free, free to go with someone else, free to be reproduced, free.