IE NetRenderer will show your page in IE 5.5, 6, or 7, to give Mac and Linux-based designers a quick sanity check. It’s free and quick.
BrowsrCamp is designed to show off how your page looks under the various Mac browser out there. You can view Safari screenshots for free, or take VNC control of a live Mac loaded with 11 different browser for prices starting at $3 for 2 days.
Browsershots is the most comprehensive of the free services, offering screen captures of your site under dozens of different Linux, Mac, and Windows browsers. It’s also popular, and it can take quite a while for requests to work to the head of the queue to be rendered. They’ve recently introduced a $15 per month priority processing program to jump requests to the head of the queue.
BrowserCam offers an online interface for managing screenshots across a variety of recent browsers and operating systems including Linux, Mac, and Windows; their coverage is excellent. You can get a 24-hour free trial; after that, plans range from $19.95 for one-day access to $399.95 for a full year. Their separate Device Capture service is the only one I know that will do screenshots of your site on Blackberry and Windows Mobile devices.
Litmus covers a variety of Windows browser, from IE 5.0 up to the alpha of Firefox 3, and say they’re adding Mac browsers soon. Results are returned via screenshots. They also integrate bug tracking, version management, and private URLs you can use to share compatibility results with clients. These features come at a price after the initial 30-day trial: 39 euros monthly for an individual account, or 129 euros monthly for a team account.
Browser Photo comes from Keynote NetMechanic. They promise screenshots from Windows, Mac and Linux systems for a $15 one-time fee or $150 per year, but don’t offer any details on which browsers or versions they cover in advance of sign-up, or any trial program.
BrowserPool takes the VNC approach to provide access to Mac, Windows, and Linux systems. Their pricing starts at 29.99 euros for one month, but unfortunately their software versions are a bit out of date.