If you've used a computer you probably have used instant messaging.
Instant messaging is great for chatting with all your friends online.
When it comes to clients for instant messaging there are tons for Windows but few for Mac.
The big one many people used is "Windows Live Messenger". On Windows it's great... on Mac, not so much.
If you are used to "Windows Live Messenger" and like it's features then you may want to try aMsn.
The way it is set up is much more similar to Windows Live Messenger. It also has most of the features of Live Messenger like video and audio chat.
Perfect, so what's the downside?
Well aMsn does not perform as well as Adium. There is yet to be a stable release of it to come out. Updates for aMsn are also much less frequent so if you have a problem you need to switch or wait a while.
The next option is the one I use. It's called Adium.
Adium is truly a Mac instant messenger. It is very similar to iChat without the Video and Audio chat.
This sucks because you can't use the lovely iSight camera that comes with all (other then the mini and pro) Macs.
On the bright side though, Adium is much more stable, gets more updates, and is the MOST customizable instant messenger client out there for Mac.
You can change almost everything about it. The colors, the emoticons, the message styles, the window styles, the icons... almost everything.
Their site has plugins, emoticons, sound sets, service icons, status icons, apple scripts, message styles, window styles, dock icons and other things.
You can download hundreds of things to customize Adium and make it however you want.
The only edge aMsn has on Adium is the Video and Audio chat. Although these are really cool features to have I would say, survive without them for now. Adium is currently working on bringing Audio and Video chat to Adium. It shouldn't be long so keep your ears open.
So there you have it, aMsn and Adium. Both great, I'll leave it up to you to decide.
Adiums Website: http://adiumx.com/
aMsn Website: http://www.amsn-project.net/index.php