It is hard to count how many times I’ve scrolled through Scott Pakin’s The Comprehensive LATEX Symbol List (pdf) looking for the LaTeX code for some obscure mathematical symbol.
That pdf document may no longer be the first place I turn.
Daniel Kirsch, a math student from Münster, Germany has created a very useful website for this purpose called Detexify2: LaTeX symbol classifier.
All you do is use your mouse to sketch the symbol that you want, and the website gives you a list of possible symbols that match your drawing (along with the LaTeX code and any font packages that you may need). It looks like it has a genetic algorithm running in the background, because it is being trained by your inputs (if do the courtesy of clicking on the symbol that you had in mind).
I found it remarkably accurate. Nice work!
Now they need to put this in as a TeXShop menu item.