I dont use IM but I always see the program Trillian mentioned.
Im not 100% sure what it does but I do know it uses lots of different IM apps in one useful program.
Take a look http://www.trillian.cc/
Yep. There are a few software interfaces which put all your buddy lists together and let you communicate through one portal to all your buddies whichever service they're using. I found a few here, , here and here, but it may be worth checking out more download sites (www.download.com, www.tucows.com etc.) for others.
Regardless of what you ISP is you can have AOL, Yahoo, MSN messangers. There is ICQ too. I have all of them but generally just have MSN and Yahoo open.