(Melissa is currently a work in progress; the stitches around her cranium haven't healed yet and I have yet to tighten the bolts on her neck. As a result, you may find that various bits of her don't work very well, and she's certainly not very polished or bug-free. Be kind.)
Melissa is mostly written in Lua, with a simple binding to the libpurple APIs; all the actual work is done in pure Lua code. Melissa's brain is a reimplementation of the MegaHAL code in Lua, done by me, which has then been generally hacked around with and, probably, broken; Melissa's responses don't seem as coherent as MegaHAL's, but that could simply be due to different training material. I'm vaguely on the lookout for a suitable replacement brain. (I may be able to steal one from Alex the Parrot.)
The program is GPLv2 licensed.