A number of instant messaging programs are available for OS X; AOL Instant Messenger (or AIM, the protocol used for Apple's iChat), ICQ, Yahoo Messenger, and MSN Messenger are among the most well-known ones.

Each of these programs can be run as a separate application, at once, on your Mac with no issues to speak of, but wouldn't it be nice to be able to have them all within one app? Or, if you have more than one account with one of the services, would it not be handy to be able to have them both signed in at once?

A few applications have come to the forefront in this category, namely defaultware's Proteus and the Open Source programs Fire and Adium X.
After using each for some time, I have come to the conclusion that Adium is the best choice for new and power users alike who want to take control of their chatting, as well as being the most stable application with the most well thought-out user interface.

Adium lets you use one program for all your messaging needs. It will let you have all your contacts on one unified list, using labels, rollovers and icons to provide feedback and information.

Setting up new accounts is a breeze, and Adium's preferences let you choose on a per-account basis whether or not to autoconnect when the program is opened.

If you have your Mac on a network of any kind, wired or wireless, where there is another Mac running Adium or iChat, you will see that user appear within a new area on your contact list. The Rendezvous (OpenTalk) protocol Adium uses is the same one you'll find in iChat.

Proteus and Fire both do a good job for the most part, but neither comes close to the stability and number of options and features found in Adium. The fact that it did not crash once, while both the others have been known to be somewhat unstable, combined with a well thought-out contact list structure make Adium X the best choice overall for multi-protocol messengers.