Anyone who works with LaTeX knows how time-consuming it can be to find a symbol in symbols-a4.pdf that you just can't memorize. Detexify is an attempt to simplify this search.
Just draw the symbol you are looking for into the square area above and look what happens!
The symbol may not be trained enough or it is not yet in the list of supported symbols. In the first case you can do the training yourself. In the second case just drop me a line (danishkirel@gmail.com)!
You could spare some time training Detexify. You could also look at the source on GitHub and if you want to contribute you're welcome. If you really, really like this you can also donate.
Detexify relies on HTML5 features so you need a fairly recent browser. Opera 9.6 / Firefox 3.5 / Safari 4 should do the trick.
Philipp Kühl had the initial idea and Daniel Kirsch made it happen.