You should check out RTcmix (Real-time Cmix). I saw a great livecoding demo by one its developers, but I haven't been able to find any videos online.
I've been using RTcmix with its Python front-end module to experiment with algorithmic composition. It's very easy to use and has nice results.
EDIT
I've been investigating livecoding further recently, and RTcmix is not intended for this, but in some cases, it can work. It's much better for pre-processed scores. Sorry for any confusion. :)
