Typical time stretching: Expensive software, rigid controls, wait for calculations.
TimeToy: Free or US$17 'Pro' version, flexible controls, real-time, mic input.

Now you're talking. TimeToy can be fun as an improvisatory
sound-warping tool or used live in performance; use rubber band-style
controls to plot time manipulations, or (with the Pro version) even
scrub and warp live. A microphone input lets you work directly with
incoming audio signal.

Mac OS X-only. (Via MacMusic)