In addition to the F/OSS Solutions that a multi-platform, there are also certain solutions that are targeted specifically for the OS X platform. Here are some of the ones I use(d).

Adium - Now we had Pidgin, which made our multi-platform list, so why do you ask do I have another multi-protocol IM client on my OS X list. Well to be honest with you, because I think Adium is a far superior Open Source project and product. Adium is extremely extend-able with tons of add-ons that people write for it to allow it to interact with what has got to be the largest range of protocols and services of any other client. One pain point with Adium, it lack video chat support so if you are into video chat, this is probably not a good solution for you.

Fink – This is probably categorized more as a “Project” than an actual application. This could be a tough one to explain to the novice but the goal of Fink is to it essentially compile Unix Open Source software to the OS X platform (also known as “porting”). This one project really allows OS X to extend its Open Source library offering bringing some truly powerful application to the OS X environment.