decompile TeX symbols
Here is the best TeX gadget ever: Detexify.
It is basically a decompiler for TeX symbols: you draw them using the mouse and you get back the corresponding TeX markup. It (claims to) supports all of the comprehensive LaTeX symbol list, so that you can avoid browsing through it again and again in the future (and keeping various printed copies of it: at home, in the office, ...).
I've used in the past some other drawing-based search engines, but I've always found them no more than toys. This is the first service that I find very useful based on similar technology. Also, source code is available from GitHub (which is unfortunately down at the time of writing, so I can't check the license ...).
Note that the web interface relies on some HTML5 elements (e.g. canvas), so you'll need recent browsers such as Firefox/Iceweasel 3.5 to use it.
(Thanks to Roberto Di Cosmo that pointed me to the service.)