The current IE7 B2 Preview is stable for website testing
The IE team announced that the IE7 B2 Preview build available online is ready for use in testing site rendering. The feature list frozen, and the rendering engine is solid and won't change between now and release anymore.
That means it's ready to use in testing site compatibilty. Star hack begone!
UPDATE: I tested running it in standalone mode using the IE7 Standalone Launch Script. It works, in that it allowed me to browse sites in IE7 without messing up IE6 or my default browser association. However, I noticed high CPU usage and had to close IE7 using the task manager - it didn't respond to that little red X thing. I believe these are minor inconveniences compared to running a separate virtual machine just to check how a site looks. Obviously, this build of IE7 just came out yesterday so I haven't put this through rigorous testing. Unfortunately, it appears that the standalone browser builds from Skyzyx have been pulled.