AOL
IM - The feature set is not outstanding, but you might
have to use AOL IM if your friends already use it.
ICQ
- The first and most popular instant messaging software.
Now owned by AOL.
MSN
Messenger - Part of Microsoft's quest to own you and
your computer. New versions tie into more Microsoft programs.
Features: Make phone calls, send a page through IM, see
when someone is typing a message, file transfer, voice and
video chat and more.
Yahoo
Messenger - Yahoo's IM app has a ton of features, such
as: multiple platform support, voice chat, integrated alerts,
international calling, webcam support, the ability to add
an IM connection to webpages and more.