I use Adiumfor Instant Messaging on my Mac simply because its flexible, powerful and ridiculously easy to use.
However, Trillian is another option for those seeking to connect over multiple networks. Trillian has been available on Windows for quite some time now, but the developers have now released it for Mac. Firstly, note that it only works on Intel-based Mac, Mac OS X 10.6 or later so PPC users or those on older OS X versions are out of luck - at least for now. You also need to sign up for a Trillian Astra Account which is quite lengthy and annoying.
Once you've got through the registration however, the biggest problem you will have is adding a new IM network. It should be as easy as clicking the "+" symbol in the client window but its not - this only allows you to add a new contact or a new group. As a result, I found it impossible to add any of my favorite IM networks such as MSN and Google Chat. Try to access the Help file and you get the message "Help is not available for Trillian".
Apart from that, the interface of Trillian is at least very simple. However, there is no support for video or audio and I can't see any reason why anyone would really want to change from Adium to this - not at this Alpha stage anyway.
