Snow Leopard upgraders should be getting their discs today, but if you're still concerned about whether or not your VIPs (Very Important Programs) will run on Snow Leopard, Apple's got an official page dedicated to incompatible software in 10.6. What's the deal, exactly?
When you install Snow Leopard or migrate to Snow Leopard, known-incompatible software is moved to a folder named Incompatible Software on your hard drive.
Snow Leopard also prevents known-incompatible software from opening. If you see an "Incompatible software" message, contact the software's vendor or visit their website for a later, compatible version.
Restricted software includes Parallels, EyeTV, and even a few of Apple's own applications, like Aperture and Keynote (albeit old versions of them). Check the previously mentioned application compatibility wiki for a more robust user-generated list of compatible/incompatible apps, and—if you've got a must-have that's not Snow Leopard-ready—let's hear about it in the comments.