A new version of Trillian for Mac has been released that boasts a large amount of optimizations and bugfixes. In addition to the fixes, the Twitter engine in Trillian for Mac has been upgraded to support native RTs by default, userstream, and a bunch of other cool enhancements.
Trillian is a fully featured, stand-alone, skinnable chat client that supports AIM, ICQ, MSN, Yahoo Messenger, and IRC. It provides capabilities not possible with original network clients, while supporting standard features such as audio chat, file transfers, group chats, chat rooms, buddy icons, multiple simultaneous connections to the same network, server-side contact importing, typing notification, direct connection (AIM), proxy support, encrypted messaging (AIM/ICQ), SMS support, and privacy settings.
Trillian (v.5.1) for Windows is available here.