www.meebo.com - online IM
I used to use Trillian, but always hated when I was away from my PC...with meebo I hit a website, log in and all my IM buddies are right there...no matter if I am on my PC, the library or wherever.
If you have two IM names (say one for work and one for personal) on the same network (AOL, yahoo...) you can log in to both at the same time.