mathworld is an excellent expanding encyclopaedic resource covering almost anything you could want to know about mathematics.

even if you don’t give a toss about maths go and have a look at the report on the copyright battle that has kept this site off line for the last year. it’s frightening.

basically a publisher offered to publish a snapshot of the website, did so, failed to sell many books due to what sounds like the failure of their own marketing, and then decided this was because people were reading the website for free and quoshed the website. the fact that much of the website was composed of user contributed material that wouldn’t have been able to grow without the website didn’t enter the publishers head.