From Sony Creative Software:
Acid is a loop-based music production tool from Sonic Foundry. Features include unlimited loop tracks (based on system RAM), automatic matching of loop tempos and pitch, real-time tempo adjustment, tempo maps, 16- and 24-bit audio support, DirectX Audio plug-in support for real-time effects, and support for nonlooping audio.
Processed audio can be directly output as WAV files or exported as digital audio tracks. Acid is also compatible with Sonic Foundry's CD Architect and direct-to-digital audio CD programs. Acid includes a library with hundreds of loops, with additional loop libraries available.