Have been an ICQ user for years now and have always rated it very highly.
And ICQ 2000b is even better than the earlier versions. One, it is a much smaller file to download -- and when you install it all your earlier ICQ database files are still in use.
Two, the SMS messenger is fabulous. Granted, there are lots of sites which offer you free SMS Messaging, but if ICQ is your main messenger program you can be sure that you’ll never miss any replies to your SMS messages.