In part of my line of work - web application development - you tend to develop software that needs to work (properly) on an awful lot of different web browsers. If these all adhered to web standards, a simple run through the W3C Validator would suffice. Unfortunately life sucks, and as such there is a need to test all sites on as many browsers as possible.
I use virtualisation to run a bunch of windows browsers in their native environment. However, testing this way took an awfully long time, as my desktop computer had a "mere" 1.5Gb of ram and only a single (Athlon64) CPU core. By the time you add GNome, Evolution or Thunderbird and Firefox to this, not a great deal is left for virtual machines.
What with DDR2 ram prices being pretty much at an all-time low and CPUs not being far behind in cheapity (wrong, but sounds nice) I decided it was time for some new tools of the trade.