Startly Technologies, LLC (formerly CE Software) produces QuickConference (formerly "Prairie"), an instant messaging system that is geared toward business use.
Servers are available for Windows and "Classic" Mac OS, and clients are available for Windows, "Classic" Mac OS, and Mac OS X. Pricing starts at $119.95 for a 5-user bundle. Downloads are in the form of 30-day demonstration versions.
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Also, if you have an older Mac, be sure to check out the "Classic" applications page for more options.
Finally, take a look at ALEMIA if you think you know that name of an application, but aren't quite sure.
These are applications that are newer and of potential interest, but which I haven't yet selected for permanent inclusion. Have a look, and let me know if you think they deserve to be part of the permanent collection!