Trillian Astra beta goes public, multi-protocol chat for everyone
The application has been in private testing for what seems like forever, but a few months ago the company started giving out beta invitations like there was no tomorrow. So removing the "private" label was probably just a formality.
Trillian Astra is available for Windows 200, XP, Vista, or 7. The company is working on Mac, iPhone, and web clients, but those aren't available to the public yet. You can sign up to be notified when those versions are available at the download page. Or you could just use Pidgin or Adium.