Despite the fact that no one seems to make any money from them, instant messaging services appear to be here to stay. With tens of millions of subscribers (some services claim more than 100 million registered users) and millions of daily users worldwide, instant messaging has worked its way into the psyche of computer users and the mainstream public (in the form of telephone-based text messaging).
The following free instant messaging services are among the most popular choices for users: AOL Instant Messenger (AIM), ICQ, MSN Messenger for Windows, and Yahoo! Messenger. Each service has its disciples, and Microsoft and AOL have spent quite a few dollars on legal fees, each trying to get other companies to play the instant messaging game on its turf. Users of one service can't communicate with users of another service, so some people install more than one client on their computer and run them simultaneously.
My personal experience is with AIM and MSN Messenger. Some client sites use AIM daily to communicate with coworkers, and I find the service useful for receiving messages from someone who's three or four time zones away. MSN Messenger is an integral component of Windows XP and is a vehicle for many of the OS's whiz-bang features. I often run the AOL and Microsoft clients at the same time, and I need to remember which service has which contacts associated with it.
Cerulean Studios' very useful Trillian application doesn't solve the problem of intercommunication among various services, but it lets you run one instant messaging client to connect to the four services I listed earlier. Trillian includes an Internet Relay Chat (IRC) client for those of us who still use the granddaddy of Internet messaging services, the IRC network.
Trillian provides a skinnable UI with more than 50 downloadable skins, support for the file-transfer capabilities of various protocols that different messaging services use, quick turnaround when service vendors make changes that break the Trillian application, and an intuitive UI that uses both common menus and right-click context menus. And for all these features, the cost is reasonable--Trillian is free, although Cerulean Studios accepts donations. I've been using Trillian for about a week, and the biggest problem I've encountered has been learning which sounds the application makes to announce various events.
If you want to try Trillian and your system runs Windows XP, here's a tip. If you've configured MSN Messenger, you need to make sure you prevent MSN Messenger from running before you launch Outlook XP or Microsoft Internet Explorer (IE). If either Outlook XP or IE is running, you can't exit the MSN Messenger client. You need to exit MSN Messenger before you launch those applications; then you can run the Trillian client without having a disconnected MSN Messenger client in the system tray, too.