Among the few applications I use constantly is my IM client. Just like my web browser, I constantly seek better and faster IM applications.
With browsers there are a few big ones and everyone knows them, for IM on Mac however, you have to take a close look to find something that suits your needs.
Since I've done a lot of this searching, I'm going to do a list of my favorites. If I missed your favorite, pleas leave a comment.
One of the most used IM clients on Mac. Adium is an application based on the same technologies that power the popular Pidgin client, but offers a native Cocoa interface and a wealth of plugins and customizations, listed at their extras pages. The 1.4 beta supports some additional cool features, but due to a lack of manpower is still not done. I recommend using the beta, as it is quite stable.
The chat client included with Mac OS X, it supports only the XMPP and AIM protocols, but I consider it one of the best if you don't need other protocols. It is possible to use other protocols via a XMPP transport, but this very is unreliable at times.
Is a well known Windows client that has recently released an alpha version for Mac. It is not very feature rich yet, but it looks and works nice so far. One downside of Trillian is that it stores your credentials in a central account on their server, one of the upsides of Trillian is that it stores all your credentials in a central account, which is cool if you use Trillian on multiple computers/phones.
This app is mainly aimed at Linux and Windows users, but with a little help of my previous post you can get it to work on Mac quite nicely. It uses the same technology as Adium, but due to a broader audience it might offer some features not currently present in Adium. Only use this if you know you need it, Adium has a nicer interface on Mac.
Contrary to what you might think Yahoo also supports WLM, provides a good Mac look and a full set of features, unlike some other branded clients. It's not the ideal thing though for the real IM diehard.
I'd go with Adium if you're a regular user, or iChat if you don't need any of those networks.
For the experimental people, Trillian and Pidgin might be good options.
For the social network people it might be worth waiting for Raindrop or Digsby.
For the WLM oriented minds, the official WLM client, Mercury and aMSN might be of interest.