They'll probably start with basic cross-platform office software (e.g. OpenOffice) and tools that would be useful to game modders (graphics, sound and 3D packages).
My prediction for a starter app is a basic teleconference app. Steam already offers API for friends lists and authentication.
Maybe Teamspeak, since that's the system of choice for Steam users.
I thought Mumble was much more popular? Either way, a Steam-based/integrated voice chat app (i.e. much better than their current voice chat options) would be one of the best Steam features.