I use Windows Live Messenger (formerly called MSN Messenger) all the time. My dh and I met on ICQ, so, while we don't use it anymore, I keep it on the 'puter for sentimental reasons. LOL Because the girls use Yahoo Messenger when they visit, it's on our machine, too, but I rarely use it.
If you're using Windows XP, Windows Messenger is
already on your machine. I suspect a lot of folks don't realize that. It may be on Win98, too, but it's been so long since I used 98 that I don't remember. There should be an icon for it on your "All Programs" list. The icon looks a lot like the MSN/Windows Live icon, so that can be a bit confusing.
For folks who just want to try IMing or who don't want to install anything, the four IM services I'm most familiar with all have
web-based versions that you can use from their websites. They're more "no frills" than the downloadable versions, but they work fine for basic chit-chat. They're handy for traveling, too, since you can IM your buds even if you're on a computer that doesn't have an IM program on it.
The ones I know about are
AIM Express,
MSN Web Messenger,
ICQ2Go, and
Yahoo! Web Messenger.