A testlab service
When I was working at a company building websites (sorry—marketing instruments :) being, amongst other things, the guy in charge of a small test lab, my brief was to test sites under development in a small range of browsers and OSes. The range was somewhat limited by available time and hardware resources, even though I was using VMWare.
This service could have helped out there: Browsercam. Not to be used throughout the development cycle, but definitely at those points where the cross-browser clause would be put to the test, this is an excellent alternative to having a battery of machines or some fancy Virtual Machine setup. At their monthly subscription rate, it is a steal.