Qnext (www.qnext.com) is a free, universal IM client written i Java. I've only tried it once, over a 3 day period. Like it, but decided Gaim was more to my liking.
I've tried Gaim several times, because of so many positive reports. I have to say, that I don't like Gaim (on Windows). Why? * It is too big (compared to Miranda-IM for example) * It uses GTK, which looks bad in Windows * Non-intuitive (read BAD) interface If you need IM client, go for Miranda.
I use gaim and Trillian (trillian.cc) alternately, but I've found gaim to quite good.
Although I use and like Gaim (especially it's plugin architecture which allows for encryption among other plugins), Gaim is not available for Mac OS X. I'd suggest Fire (http://fire.sourceforge.net/) or Adium (http://www.adiumx.com/) both of which support multiple protocols. Also don't forget Apple's iChat (http://www.apple.com/ichat/) -- also free, but it doesn't support multiple IM types (just AIM).
I recommend SIM (Simple Instant Messanger - http://sim-icq.sourceforge.net) It's GPL-Software.
Gaim is now called Pidgin. Version 2.0 is available for download @ http://www.pidgin.im/ for Windows and Linux.
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