Gaim is a popular opensource IM utility. I use Gaim to communicate with my friends using various protocols like Yahoo/MSN/IRC etc. It works both in Linux & Windows (also in BSD, MacOS X )
For typing tamil in any windows program, I use a keyboard driver software by name ekalappai (which is also created by me). But somehow ekalappai cannot be used with Gaim. It simply does not work. So far no time to debug this. So whenever I use Gaim in windows I type tamil text in notepad then copy & paste in Gaim and send it.
Now there is a solution to this problem. Anna University student Varun has created a plugin for Gaim transliterate english text typed in gaim message window to tamil.
You can read about this plugin in Varun's blog.
Congrats Varun !