What we are looking for is SIPE support (possibly a special implementation of SIPE but I'm not sure). Don't confuse this with SIP/Simple as they are not the same thing. There are two options I found. The first is pidgin, an open source IM client. The second is called Adium. As I understand it Adium is a build of pidgin for OSX. While neither comes with SIPE support out of the box both have projects developing SIPE support in the form of a plugin. I recommend using Adium for its simplicity but either should work.
You will first need to get a copy of Adium from here. At the time of this writing Adium 1.5.3 is available and works just fine. Download and install the application on your mac.
Next you will need to go here and download the latest version of the SIPE plugin. Many thanks go out to jvoitek who packaged this up for us. Once you have downloaded the zip, decompress and run the plugin. This will add it to Adium.
Finally you will need to run the following two commands from the terminal.
cd /Applications/Adium.app/Contents/Frameworks/libpurple.framework/Versions
ln -s 0.10.5 0.10.0
Now that the dirty work is done you can start up Adium. You will notice that you have a new account option called Office Communicator. From there its a simple matter of filling in your exchange info and vuala. You now have a Microsoft Office Communicator alternative.
If you have any issues I recommend referring to this forum discussion of the topic. I hope this helps folks.
