From the makers of PowerStrip comes this handy little utility:
iRotate provides convenient access to the native rotation capabilities present in contemporary display drivers, via a popup menu accessible from the system tray and optional system-wide hotkeys. It's no longer necessary to resort to bloated Windows hacks, additional software layers or phantom drivers to achieve content rotation. In most instances, support is now available directly from the graphics chip manufacturers, who continuously improve and apply quality assurance to their drivers.
By leveraging the native rotation capabilities now provided by ATI, nVidia, Intel, Matrox, S3, XGI and others, iRotate offers exceptional speed and efficiency, with minimal impact on scarce system resources - the entire iRotate package, including installation, documentation, and native language support in all the major European and Asian languages, weighs in at only 125kb. And like all EnTech graphics utilities, iRotate supports multiple graphics cards from various vendors, simultaneously, under every multi-monitor enabled operating system from Windows 98 to Vista.
... and indeed Windows 7.
iRotate is freeware.
If you want to "rotate" the mouse as well, you can also try this:
EeeRotate - A tiny program to rotate your screen and touchpad or mouse at the same time. Especially designed for EeePC but should work with all computers.
Ctrl + Alt + Right: rotate screen and touchpad or mouse by 270 degrees
Ctrl + Alt + Up: back to normal
EeeRotate conveniently combines iRotate with SakasaMouse.
EeeRotate is freeware and can be used portable.
Edit: i just tested it with an ATI card, it works.