The best replacement for trillian is pidgin.

The best alternatives to trillian are rated as following:
★★★★★  pidgin
★★★★★  digsby
★★★★  adium
★★★  qip
★★  miranda
★★  gaim
★★  aim
★★  trillian pro
★  meebo
★  imo
★  omnisphere
★  im+
★  trilogy
★  nimbuzz
★  instantbird
  ebuddy
  beejive
  yahoo messenger
  icq

Your alternative for trillian is not listed? Suggest!
Speaking about trillian, one probably shouldn't forget about google talk.

Other search terms related to trillian and their ratings are:
★★★★★ google talk
★ icq
irc
gmail chat
You have in mind a companion for trillian and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of trillian in all years. Click on graph for more.
trillian
pidgin
digsby