Tools I've actually used
While not serious mathematics, I have written computer programs collaboratively in real-time with
- Skype(Voice) and a cheap USB headset to have my hands free
- Etherpad (Real-time simultaneous document editor)
- Custom shell script to feed the etherpad document right into the compiler
Works perfectly for this particular purpose. In particular, it's much better than two persons trying to program in front of a single computer.
I've also bought a pen tablet (a small Wacom Bamboo Fun for about 90€) that I am using for mathematical illustrations. Being able to sketch pictures is astonishingly liberating when communicating mathematics on the computer!
Tools I'm planning to use
Of course, I'm now trying to use the tablet for real-time collaboration on mathematics. I haven't found a good modus operandi yet, though, apart from some experiments with the CoSketch online whiteboard. Usually, the main problem is that the colleague doesn't have a pen tablet himself…
I'm also looking into the possibility of setting up an electronic whiteboard with a Wii remote. But that would just be a substitute for a pen tablet.
Ah, and for sharing a set of documents with your colleague, there is Dropbox, which allows you to synchronize folders on different computers. No more chaos with different versions in different Emails!