There are various feature-packed alternatives to the default AOL Instant Messenger client for Mac OS X.
When people started discovering AOL's Instant Messenger taking up a large portion of their CPU for seemingly menial (or nonexistent) tasks, the proverbial chad hit the fan; people wanted something better and they wanted it now, regardless of what silly or pointless features they may end up missing. Let's take a brief look at some of the alternatives that have gained popularity:
Since Apple ships iChat with its OS X 10.2, there's a very good chance that iChat will become the reigning champ of AIM imitators. Created with the blessing of AOL itself, iChat supports chats, file transfers, and buddy icons and uses a GUI that manifests messages into cartoon-style balloons (which don't look as bad as they sound). It has strong support for Mail (a column that tells you if the sender of the email is online) and the Address Book, along with the ability to customize which actions will be performed for a number of common events (buddy logging in, buddy typing, etc.).
Adium has been gaining a strong following for guerrilla AIM users, mainly due to its immense customization and its low system requirements. It has a clean and refined interface, as well as being minimal for those worried about screen real estate. It's free and is built using Cocoa (and thus gets a number of things for free: spellchecking, antialiasing, etc.).
Fire is the granddaddy of instant messengers for OS X; it's been around as long as OS X has and has consistently been improved from version to version. It's open source and it shows, offering langauge localizations contributed by others, hearty amounts of bug fixes, soundsets, icons, and more. While its interface may not be the prettiest, its ability to hook into AIM, Yahoo!, MSN, ICQ, Jabber, and IRC is a clincher for those with multiple service needs.
Proteus, my current favorite, is much like Fire in that it hooks into a number of providers: AIM, Yahoo!, MSN, ICQ, and Jabber. It benefits by having a strong amount of customization, with the interesting ability to run shell scripts at various points (such as when messages are received, so that, for instance, you could make your own logging system), along with the ability to choose themes, icons, soundsets, and so on. Its GUI is a little more refined than Fire's.
If these AIM alternatives don't satisfy your messaging itch, don't hesitate to browse the excellent VersionTracker (http://www.versiontracker.com) for more possibilities. You may also want to take a look into Jabber messenging systems, which can hook into any IM protocol that the Jabber server supports. The best way to find these XML-based alternatives is to search Sourceforge (http://www.sourceforge.net/) or VersionTracker.