There is no dearth of software for instant messaging on a Mac. Apple has a strong history of supporting chat protocols on the Mac, as they have promoted the "Bonjour" protocol for relaying IMs over an Internet connection. As a result, there are many options for Mac users who want a strong, multi-featured IM application.
iChat comes with every version of OS X, the Macintosh's well-known operating system. It is, as a result, the most popular IMing application. However, it is by no means the best; for individuals who only manage one or two contact lists (Gmail, AIM, Jabber, IRC), iChat will likely suffice. The Aqua user interface is a plus, as it integrates nicely with the rest of the operating system, and that's always a good thing.
In addition, iChat provides excellent communication features for users who need to do more than IM. For example, the latest version of iChat allows users to share screens (and even, with permission, control their friends' computers), record audio conversations, and perform video chat. For users who require their conversations to be totally private, iChat provides encryption options.
iChat is the ideal Mac chat application for most users.
[Adium] is the most popular iChat alternative on the Mac, and with good reason: it supports dozens of chat protocols, including AIM, Gchat, Yahoo!, MSN, Jabber, Bonjour, IRC, Skype, and others. Chances are, if you use an IM service, it is supported by Adium. With Adium, you can combine all of your IM services into one contact list, and even sort your contacts by first or last name.
Unfortunately, Adium does not provide video or voice calling, as iChat does.
Proteus is built on the same framework as Adium is, but it takes a slightly different approach. In doing so, it provides a more customizable user interface. For IM users who like tweaking their software, Proteus is an excellent choice. However, given that it is incredibly similar to Adium, but without as large of a development community fostering its growth and development, it is probably better to stick with Adium, which is not likely to stagnate in development any time soon.
[Instantbird] is available on Mac, Windows, and Linux, and offers a different IM experience than the previous three applications. Specifically, it uses the open source Mozilla rendering library, used in Firefox and Thunderbird (hence the name) to display your contact list. If you love the way that Thunderbird displays your contact information and want to try an IM application that is off the beaten path, Instantbird is a good pick. However, it is in version 0.2, which means that it is likely that not all of the bugs have been worked out just yet.
Overall, iChat provides most of the features that most users will need. For most other users, Adium will suffice. For now, though, there isn't a compelling reason to use more obscure IM applications on the Mac. Time will tell if Proteus or Instantbird become viable competitors to Adium or iChat.