iChat is an instant message client from Apple Inc. It supports various protocols allowing you to talk to your buddies and friends from ICQ, AIM, YIM, Facebook, and Google Talk networks to name a few. The client supports Audio and Video calls, SMS, File transfers, and tabbed messages window.
The client comes pre-installed in all the latest Mac OS X systems and is typically placed in the Dock. The program's support of multiple protocols allows you to have all of your friends in one place - there is no need to install other messengers and you can use your iChat for AIM, YIM, and GTalk needs. The conversations are grouped and tabbed by the protocols. That is all AIM conversations will be tabbed in one AIM window and so on.
Audio and Video calling is another great feature of the client. There is a great number of controls and options to help you set the best performance for calls on your network and based on your computer's settings. Unfortunately, I was unable to test video quality of video calls as I do not currently have a webcam, but it is important to mention that the settings for video calls have limit options for bandwidth allocation, which is missing in all other IM clients (including Skype).
My personal opinion: seven or so years ago, when I was a hardcore Linux/Windows user I had to learn to use Macs for my position as a Technical Support staff. Around that time I had a chance to play with iChat for the first time. I was amazed at how cute, streamlined, and functional was that client. I remember when you typed a message it appeared scrolling up to its place, which was ultimately cool to me. However, it seems this version was more functional/operational rather than cool and new, and amazing like it appeared in the past. It makes sense that Apple's programmers are now working on apps for iPhone and iPad, and the new version of Mac OS X Lion and no one has resources to make iChat cool.
Sidenote: it's absolutely free but it is tied to one system (I mean the logs, settings etc). I prefer to use web services that combine all the clients, such as Meebo or Trillian - you get to keep all the logs in one place and to use it on any computer with an internet connection and web browser. The only problem with those services is that they are unencrypted, which is a huge minus, but I will live.