simrob and I had a vaguely productive time using twiddla, an online multiuser whiteboard that supports LaTeX equation entry. However it's sort of a pain to use with a laptop trackpad. I imagine a mouse would be fine for diagrams, and a wacom ideal.TeX the World is a greasemonkey script (and also chrome extension) that "adds LaTeX support" (for some values of such) for pretty much any website/chat program/gmail/etc.
For instance I can just say [;\frac{\vdash \sigma \sim_\gamma \sigma' : \Gamma\qquad \Gamma,\Gamma_0 \vdash e : \tau}{\Gamma_0 \vdash \sigma e \approx_{\gamma\Gamma} \sigma' e : \tau} ;] and get