Tessa IM
Tessa IM is a relatively new instant messaging application.
For users, Tessa IM promises to be just as good as Miranda IM while being just as simple to use as Yahoo Messenger, MSN messenger (Windows Live Messenger), or Pidgin/Gaim. It's being designed and written to be supported on all major platforms and has, at the time of writing (15 Nov 2007), two options of graphical interfaces and only a plugin for the Jabber protocol.
For developers, Tessa is written in C++ and Lua. It is modular and extensible, it uses gloox lib for Jabber/XMPP communications. It has two GUIs, the default one using wxGTK and Momple, using Qt4. It was started by the same hacker that is developing remote controlling and scripting capabilities for the UK popular Neuros OSD gadget.