Browsercam
Test your web design on any browser on any operating system. Check javascripts, DHTML, forms and other dynamic functionality. Browsercam provides services like Browser Capture, Device Capture, Remote Access & BC-Virtual. You can use free trial account for 24 hours or maximum 200 screenshots can be captured.
Browser Capture:
Provide URL & select operating system, browsers & screen resolution as per your requirement. Browsercam will provide screenshots on that specification.
Device Capture:
You can even test your site on Mobile devices.
Remote Access:
Get remote access for required specification & test your site’s functionality.
BC-Virtual:
They also provide a facility to create your own virtual machine & let you install your own softwares.
Browsers(different versions are available):
Internet Explorer, Mozilla, Firefox, Safari, Konqueror, Opera, Netscape, AOL, Camino & Google Chrome
OS(different versions are available):
Linux Fedora Core, Macintosh OS X & Windows
Cross Browser Testing
It gives remote access for testing. Trial account holder gets only 5 min of testing sessions for free.
Browsers(different versions are available):
Internet Explorer, Mozilla, Firefox, Safari, Opera, Netscape, AOL & Google Chrome
OS(different versions are available):
Macintosh OS X, Ubuntu Linux & Windows
Browsershots
Allows you take screenshots of your site in different environments for free 
.
Browsers(different versions are available):
Internet Explorer, Mozilla, Firefox, Safari, Konqueror, Opera, Netscape, AOL, Camino, Google Chrome, Dillo, Epiphany, Flock, Galeon, Iceape, Iceweasel, Kazehakase, Minefield, Seamonkey, Shiertoko & K-Meleon
OS(different versions are available):
Linux, Macintosh OS, Windows & BSD
Browserpool
Access computer via a VNC-client, you can test for free using trial account but that session can be terminated if any of their paying customer wants to use that computer.
Browsers(different versions are available):
Internet Explorer, Firefox, Opera, Safari, Mozilla, Konqueror, Netscape 4, T-Online, Lynx, Amaya, Camino, Shiira, Dillo & ELinks & w3m
OS(different versions are available):
Linux, Macintosh OS X, Windows & BSD
You may want to take a look at this new fast cross browser screenshot application.
Currently it only supports Firefox, Internet Explorer and Safari on Windows, but as it is being actively developed more features and additional browsers will be added in the future.
I would suggest using browsers offered as SaaS instead of just screenshots. You do want to know if it actualy works, not just if it looks ok. For more info see this post:
http://codingstrategist.wordpress.com/2010/06/27/what-is-spoon-net/