I used to use Browser Shots to check my sites in different browsers. While this was a good free service it did take up to 30min to get a picture (there was a pay for service but i never signed up to that). Recently i have been using browserlab from Adobe. Very nice it is too! It’s free at the moment and i hope it stays this way as it takes between 10 seconds and a couple of minutes, i suppose it depends on how busy they are.
I work on a mac and have Safari, Chrome, Firefox and i can boot up windows and check ie7. However Adobe is faster than booting up Windows. This is what Adobe say:

BrowserLab provides web designers exact renderings of their web pages in multiple browsers and operating systems, on demand. BrowserLab is a powerful solution for cross-browser compatibility testing, featuring multiple viewing and comparison tools, as well as customizable preferences. Since BrowserLab is an online service, it can be accessed from virtually any computer connected to the web.