adium
Adium is an instant messaging client which is covering a lot of protocols (no IRC unfortunately).
With iChat, the client that comes with Mac OS, I had some problems with ICQ because my chat partner got everything I wrote wrapped in HTML tags and I did not find an option to turn that off. Later I found out, that a plugin for iChat was needed to filter the HTML out of my messages but I was with adium already then and since I liked it and it's free, I kept it.

adium is open source freeware (GPL)