Atlantis is an attempt to create a Mac OS X native equivalent to the Windows 'SimpleMU' client. It supports features such as user-defined spawns, an advanced (but fairly easy-to-use) event system, SSL encryption, MCCP compression, and more. It is built around a reusable toolkit called 'Lemuria,' which provides for dynamic control of window layout and UI, and which was created for Atlantis.
Atlantis is presently in a freely available public beta for people interested in trying it out and providing bug reports; when it reaches 1.0.0, there will continue to be a free version but a $10 shareware version with certain premium features enabled will also be available.
What's new in this version:
*** This has one significant bugfix, and as such will still work on Panther and Tiger. *** GENERAL: There was a second string input lockup bug, where a server that sent Latin1-encoded text when UTF8 had been negotiated would cause the input stream to lock up. Now, if a data blob is unparseable, Atlantis will try to use a previous encoding to generate a string. This primarily affected certain MUX2 setups.