Miranda was the open source IM app that I used on a Windows machine - the great thing about it was that it enabled chat on AIM, Yahoo, MSN, ICQ and was lightweight - and more importantly - free of
spyware
I tried a few apps (including IM clients which one had to PAY for!) and in the end found the excellent Adium. Just like Miranda, Adium enables chat on multiple networks and a few more - and its free.
