http://detexify.kirelabs.org/classify.html
EDIT: GitHub sources:
https://github.com/kirel/detexify
https://github.com/kirel/detexify-hs-backend
I remember once upon a time it was faster for me to write out an equation than to typeset it. Then I met detexify and Mathematica. Both are brilliant, one is free :-)
http://detexify.kirelabs.org/classify.html
EDIT: GitHub sources:
https://github.com/kirel/detexify
https://github.com/kirel/detexify-hs-backend