If you've ever used math, then you'll find Eric Weisstein's World of Mathematics useful. It is surprisingly comprehensive, succinct, and helpful. It is the math resource.

Weisstein also created similar (though less complete) references for astronomy, chemistry, physics, and other topics - all available for free, online, at Eric's Treasure Troves of Science.

The usefulness of MathWorld is immediately apparent, and it occured to me yesterday that my field of study, computational fluid dynamics, could use such a resource. I am hereby declaring my intention to create Mouser's World of Hydro or something like that. If it takes off, I could possibly extend this to all of simulation physics.