ChucK is an audio programming language for real-time performance. It is possible to add and modify code without stopping the audio stream, which makes live coding sessions possible. I played a bit around with ChucK in the miniAudicle editor, which can be recommended – the real Audicle (a spectacular OpenGL interfaced IDE) on the other hand proved to be quite unstable.
For a more extreme version of live coding check out Quoth (now this is weird – a Zork-inspired user interface).