I personally prefer ICQ for most of my non-IRC chatting for two big reasons: 1) ICQ is flexible, and 2) Everyone I know uses it. I also use AIM, but for the most part ICQ is the standard. Yahoo is nice if you want a slim and very basic IM setup, or if you need to talk to other people you meet on the Yahoo games site, but I think ICQ is the better of the IMs out there.
An open, cross-platform journaling program.
A scientific plotting package.