I came across MyScript yesterday, and have been very impressed. It's an SDK which can recognize handwritten math (among other things). It has correctly recognized everything I've thrown at it so far, even though I'm using a mouse so my writing's pretty crappy.
I'm seriously considering trying to build something with the API. I'm not sure what; I just want to play with the cool technology. Perhaps an app which gives you an algebra question, then uses math recognition to interpret your working. It could then see that you made a mistake on step three (for example).
Perhaps I've just late to the party, but I'm still astounded that this is possible.
Windows comes with a dedicated math handwriting input panel. Totally random, all because of their long (failed) push for Tablet PCs. I'm not sure how well it works though.
It needs a lot of massaging, at least for my handwriting. But it's been a while that I tried it with an actual pen. The main problem is that it recognizes strokes, so if you go back and close a gap in a symbol to make it appear right the recognizer will think you changed o to σ or something like that.
Back in uni I was able to take math notes quite well by typing in Word. The math notation there is similar enough to LaTeX (including most macros for symbols) but types much faster. But that's not handwriting, admittedly.
