You can purchase Max/MSP and Jitter together. Max/MSP is a graphical programming environment, which means you create your own software using a visual toolkit of objects, and connect them together with patch cords. The basic environment that includes MIDI, control, user interface, and timing objects is called Max. Built on top of Max are hundreds of objects, including two powerful collections from Cycling '74:

• MSP, a set of audio processing objects that do everything from interactive filter design to hard disk recording.
  
• Jitter, a set of matrix data processing objects optimized for video and 3-D graphics.