Having been caught out recently when a web site I launched displayed perfectly on IE, Firefox, Chrome and Safari on Windows but was corrupted when viewed using Safari on the Mac
Litmus may help you. It will take screenshots of your webpage(s) in a wide variety of browsers so you can make sure that your site works in all of them. A free alternative (Litmus is a paid service) is Browsershots, but you do get what you pay for. (In some screenshots that Browershots returns, the browser hasn't yet finished loading the webpage...)
Of course, as other people have suggested, buying a Mac is also a good solution (and may be better, depending on the kind of testing you need to do), because then you can test your website yourself in any of the browsers that run under Mac OS X or Windows.