Silverback is a guerrilla usability-testing app for people who design and build websites.
With Silverback, you can run your own low-cost, unobtrusive usability tests using the hardware you already have in your Mac.
Silverback 2--available through the secondary download link above--is now available as a free but unsupported product in order to support older machines and operating systems (OS X 10.4-10.9).