Detexify2 is a neat web application that lets you draw a mathematical or logic symbol with your mouse and then it will tell you how to make that symbol in TeX or LaTeX. (It learns from other users’ inputs.) So if you’re a LaTeX user who can never remember how to make a turnstile (vdash) or a definition symbol (triangleq), then this is probably useful for you. I suspect it would also be useful for identifying some name for an unfamiliar symbol even if you have no intention of using it in a LaTeX document.
(Via Geoff Washburn.)