GTalk on a stick
Gee, it works like a charm, and boots up faster than Skype on a stick. Just thought you ought to know.
UPDATE: A couple of people have asked, so just to be clear: I'm talking about launching it from a USB stick (as an alternative drive) onto a brand new PC. The first time I tried it I was prompted to log in, but this took no time at all (much faster than Skype in a comparable setting). The second time, I happened to be logged into Gmail already, so it opened up immediately and was already displaying presence information from my contacts list.