I use these tools often for communicating with people quickly and efficiently. I have found these tools to be very helpful over the past few years as the organizations I have worked with have been pretty much virtual. Many are, particularly development organizations delivering professional services to multiple clients.
I use Trillian often as it supports multiple IM clients simultaneously. I have used GAIM too, which actually has a nicer interface but functionally I still prefer Trillian. For one thing I like the fact that it automatically captures chat history. This can be helpful if you want to review notes later on.
I use Skype quite a bit too. There’s nothing better if you want to talk to a developer in real time to get updates or project status. The quality is good, even on my laptop. The thing I like about IM clients in general is the idea of presence and availability. This really cuts down on the phone tag routine.