Like TiK, TNT is an open source AIM client, but whereas TiK is written in Tcl/Tk, TNT is written in elisp, and hence runs within emacs. TNT of course stands for "TNT's Not TiK".
Strictly speaking, both TNT and TiK (and a few other clients written in other languages) are not actually AIM clients, they're TOC clients. AOL's AIM server is called oscar, and the protocol for talking to that server is also called oscar. Because AOL didn't want to publish the specs for the oscar protocol, they set up a proxy server, called the TOC server, which knows how to talk the oscar protocol to oscar, but which clients can talk to using a simplified version of the protocol, also called TOC. TOC stands for "Talk to OsCar". And AOL published the specs of the TOC protocol.
Also, note that users of TOC clients and users of the standard AIM (oscar) clients can all talk to each other transparently.
Several TOC clients, including both TiK and TNT, were originally developed within AOL, but since AOL has stopped further development, they have been taken over by the open source community. The latest version of TNT can be found at http://tnt.sourceforge.net/ .