The Twinbow II is an interesting, and beautifully designed, crossbow made by Swiss Crossbow.
It can be cocked in three seconds and fire a bolt at a speed of 370 feet/sec [ I know nothing about crossbows so I am not sure how revolutionary a 3 second cocking action is].
This video demostrates how it works the action works.
UPDATE: James has written an interesting post on how this crossbow compares to regular crossbows. It is worth reading.