Adium

I've been using a Mac for 12 years now. I'm pretty familiar with most aspects of the Mac. I've also been fixing Windows boxes full-time for 2 years now. I'm pretty familiar with most aspects of Windows as well (sadly, in some cases, I'm more familiar with Windows than the Mac.)
I'm going to start listing my favorite software on a regular basis and tell you why I use it. I hope that this will be a benefit to you.
{Before I get going though...one warning; I'm really, really picky. I hate ugly programs (Internet Explorer 7 & Accordance - looking your way) and awkward programs (Picasa, Flock, Thunderbird - this list is long). I will probably list some of them because they are better than the competition, but that really isn't an excuse for an ugly or awkward user interface.}
Let's start with the Mac and Adium:

Adium
type: multiple protocal instant messaging client
cost: free

pros: All IM/Chat friends are in one convenient list
- All IM/Chat messages come up in one tabbed window
- Doesn't use much RAM
- Simple setup. All of the main chat settings are saved in the setup process
- Good looking & highly customizable UI
- Free
cons: No video chat (yet)
- Minor bugs - if the app is hidden, sometimes a new chat window doesn't pop open
rating: 95% (completely subjective)