I realize that the original question asked for a virtual solution, however, as of this time there isn't a legal way to emulate a Mac on non-Apple hardware.
A service like BrowserCam is the best bet if you are entirely opposed to purchasing a Mac. Remote access from BrowserCam is about $400 a year and they have cheaper packages that may suit.
Depending on how often (and on what browsers/platforms) you need to test, you may want to consider purchasing a Mac Mini. A refurbished Mini runs between $500 and $600 and may be cheaper over the long run.
If cross-platform testing is central to your company's business, a full size Intel-based Mac running both OS X and Windows (with Parallels, BootCamp or VMWare) for testing may be a better option.