Adium probably takes the cake in the realm of iChat competitors. Although it doesn't do audio and video chat yet, these features are certainly in the works, and we can expect to see them sooner rather than later. From the looks of it, Mercury, a free but closed source app, could potentially become OSS at some point, which would add an additional client capable of video chat to the mix. Although not open source, the latest version of Yahoo Messenger touts seamless video chats.
A few features that come with Adium that give it an edge on iChat include tabs, the ability to view chat history, and a menu bar icon that you may find more useful than the normal window. Adium also weaves together all of your various instant-messaging clients into a single uniform interface. This feature alone can significantly streamline the clutter on your desktop.
Figure 4. Adium delivers tabs.
Figure 5. Adium history.
Adium ties together all of your various instant-messenger accounts, delivers tabs and chat history to your chatting experience, and provides you with a neat menu bar icon (not shown).
Other alternatives out in chat land include Skype, the well-known pioneer in VOIP, gaim (try Fink), and Fire among others. Or, if you're in the market for a slick open source IRC client, consider looking at Colloquy.
The health in this category seems pretty good overall, and from the looks of it, it'll continue to receive a few more boosts in the near future as more apps like Adium introduce video chat capabilities and other improvements. Like the web browser category, the innovation in this arena really seems too high, and this can only mean good things for consumers as Apple develops the next generation of apps for Leopard.
Overall health grade: B+
OSS health grade: B
Text Editors, Word Processors, etc.
Does anyone else out there consider TextEdit a core app? Despite very little press, it's pretty handy to have it in your toolbar for the basics. Incidentally, the source for it comes with Apple's developer tools, so it's OSS too! As you probably already know, there's simply no shortage of text editors out there. (Read here to build your own in only 15 minutes.)
Among the most noteworthy OSS apps out there for slicing and dicing text in its various forms are Vim, Emacs, AbiWord, TeXShop, and Smultron. Mac DevCenter's text.editors.addicts.txt provides a nice overview of these and several others. There's also an entire arena of paid applications that some users swear by; TextMate, SubEthaEdit, and BBEdit are some of the better known ones.
It almost goes without saying that the health in this department is excellent. Although we're bordering on office products in this category, we'll stay away from there since office products aren't included by Apple as stock apps. If, however, you are in need of some OSS office tools, you might start out by trying NeoOffice or OpenOffice.org. (You can read an overview of an older version of NeoOffice here.)
Overall health grade: A+
OSS health grade: A