Kahoots is an option, it allows people to chat with others on that website. You can also setup an IRCd and use a JavaChat (or use Dalnet/Efnet with your own channel)
If you want a true messanger you can make one. MAking a server would be the biggest chalange. I started to make one a while ago but since most my EXE coding is done in VB and not C/C++ it would be a chalange running the server. (If i could code it in c/c++ i could run it on my redhat box but don't know how.) You can create your own protocol, which I would gladly help you with, or just base it off an already known protocol.
Here's a breakdown I wrote for Yahoo Protocol [http://www.tgcashin.com/coding/yahoo_manual.php]. It's a very simple, yet effective protocol that will allow you to do alot. Personally I think AOL has the best protocol, but that is very advanced and only needed for full form building and such.
If you need any help with any of the suggestions, contact me [aim: Twos The Limit]. Also you can find full chat/im clients coded at www.planetsourcecode.com
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