MathWorld has been taken down due to a copyright lawsuit:

Three and one-half years ago, Eric signed a book deal with CRC in which he agreed to provide printed, camera-ready pages for the encyclopedia. He thought he was selling them a printed snapshot of his existing web site, not the whole web site. CRC now claims that he sold them his whole web site, not just a printed book.

I wish I knew enough about copyright law to say for sure, but Eric sounds very much in the right here. I hope the lawsuit completes in his favor soon, so the web site can be restored. See the FAQ for information, links to news stories, and other documents.

Eric's Treasure Troves of Science is not affected. Yay! Eric's sites are just the kind of thing I hope to do with my copious spare time; I can't imagine a more satisfying hobby/job.