Cumulate Draw is a web based drawing application that looks very promising. One drawback appears to be that you can't insert text just anywhere in your drawing. You have to create a box for it and set the borders and transparency. No big deal once you get used to it. Anyway, I think it could come in quite handy for those "quickie" graphs to put on exams and such.
Hat tip to Newmark's Door
A similar program is Gliffy:
http://www.gliffy.com/