Eric Weisstein's World of Mathematics, The Web's Most Extensive Mathematics Resource--and he ain't kidding. I came in looking for an article on circle packing and I just couldn't leave. A huge collection of articles, cross-referenced, with external web references for every article.
This is BrainLog, a blog by Dan Sanderson. Older entries, from October 1999 through September 2010, are preserved for posterity, but are no longer maintained. See the front page and newer entries.
June 1, 2000