I like playing with a bunch of them: Extempore, Pure Data, Grace and Euterpea (Haskell-based)are my current toys. There are many more to chose from - Openmusic, even Manx, a Forth system. I prefer to have more control, and both Sonic Pi and Overtone are wrappers for Super Collider. In Extempore you can create new ugens, whereas in Overtone and SonicPi I believe you only have available what's provided for in Super Collider. Granted there is plenty enough for 99% of people looking to livecode. I am biased towards Extempore because I like Lisp, it does both visuals, music, 'cyberphysical' programming, and it was truly built from the ground up for livecoding [1]. I am very excited about xtlang as a general purpose programming language outside of livecoding. I believe Extempore is being courted by the HPC crowd too after Ben Swift's and Andrew's work on it. AFAIK, Overtone is Clojure atop Super Collider, and Sonic Pi is Ruby atop Super Collider. They have nice interfaces, and you can do visuals in Overtone with Shadertone, a sort of ShaderToy for Overtone. I am not a Ruby fan however, and I prefer a more traditional Lisp or Scheme to Clojure. If I need functional programming in a Lisp, I like Shen, but nobody has ported a livecoding environment to it. Take a look at Andrew Sorensen here to see how Extempore can be applied to Western Music in action[2]. It has a Scheme language, and a c-like language called xtlang. It can be used for other things besides livecoding too. I started with Fluxus, but on Windows it doesn't support livecoding audio, only livecoding visuals to the audio stream you feed it. GRACE [3] is very easy to start with, and complete with built-in tutorials. It is Scheme-based, but has a more generic language called SAL. It is crossplatform, and comes packaged as one self-contained file to download and execute. It is the quickest to start with in my opinion. Jason Levine has ported the code from Daniel Shiffman's book, The Nature of Code book to Extempore's xtlang [4]. A great way to learn Extempore.
Livecoding seems to be growing more and more with a lot of hardware toys to go along with it. Exciting times.
