With respects to multi-protocol IM clients on Mac OSX,
I used to be a big user of Fire,
since it was really the only multi-protocol IM client for the longest
time.
Now, Adium comes along and starts
using libGaim and also has tabs for its chat windows so that no longer are there
many many windows popping up all over the screen.
Out of the box though, Adium is kinda annoying: it has an iChat like word-balloon
layout (which you can partially turn off) and the contact list is kinda anemic.