iChat is a great and powerful Instant Messaging client that comes standard with all releases of OS X. While it has its strengths, it also has its weaknesses.
The lack of ability to communicate with the MSN and Yahoo networks and its default way of separating even your AIM, .Mac, and Jabber contact list can cause a lot of clutter on your desktop. If you don’t use chax, you may need to have a separate Space for each IM network you’re using.
If these issues bother you with iChat, then you may be interested in Adium. A free Instant Messaging client that allows you to connect all your IM networks and keep your contacts in one contact list.
Adium has a lot of customizable features and integrates well into the rest of the OS X environment. Although Adium has some great features, it does not have Screen Sharing capabilities, solid video chat, or some of the other strengths of iChat.
Now… if they could just merge their features..