No, Trillian is currently only available as a Windows software. Trillian Astra will introduce a web-based version that can be used in a browser (and therefore also on Linux or Macintosh computers). The core code of Trillian Astra compiles under Linux and Mac OS X, but that is  just a first step  and  does not mean that a usable  Trillian version for these systems  can be easily created. 

An alternative for Mac OS X is Adium X.

An alternative for Linux is Kopete or Pidgin (formerly known as Gaim).