This one's near and dear to my heart, and I'm really hoping we can get some discussion going about it here. Of all the online collaboration tools out there, this is the oldest one that was and is successful and is still supported, although technically it never left alpha.
NINJAM is a method for online realtime collaboration. It works very well for ambient music that doesn't have tightly structured key changes, and it's based on the idea that while latency forbids literal real-time sync, a central server can sync multiple people playing at the same tempo by delaying transmission by a set number of bars. Each client can record the results, and the server can as well; this produces a fascinating set of takes where each person hears the various parts in different order based on relative latency, a sort of audio Rashomon.
Reaper actually supports NINJAM, and the main NINJAM information server these days, with builds for the server and client software for various platforms, is on the Cockos site. I have recorded two albums with it, and they've been among my successful.
This thread is open to people who'd like to try running a NINJAM server or who'd like to try using it with collaborators, as well as people who have had good luck (or bad luck) with it in the past.