In a technical note, Apple lists third-party programs that are restricted during installation (what Cupertino calls "migration") and others that it knows will crash on opening. Ironically, some are Apple apps.
In Mac OS X v10.6: About incompatible software, the company offers two lists: one that the Snow Leopard installer will isolate in a folder titled Incompatible Software; and another that Snow Leopard will prevent from launching.
Software restricted from opening
After installation if you try to open ("launch") software known to quit unexpectedly, Snow Leopard will prevent the application from opening and you will receive a dialog notifying you of the incompatibility. If you see such a dialog on your Mac, use the table below, then check with the software vendor to see if any Mac OS X v10.6 Snow Leopard-compatible updates are available.
Some of the restricted apps are older versions, even of Apple products. However, some users won't have updated their software and should check the list before upgrading. Some of the listed apps and drivers on the list may be important to your workflow.
Here are a few that caught my eye (in eyeball order):
Silicon Image SiI3132 drivers (these are the drivers for ExpressCard storage such as eSATA drives)
Parallels Desktop 3.0
Elgato EyeTV Versions 3.0 & 3.1
Apple Aperture Versions 2.1.1 and earlier
Keynote Versions 2.0.2 and earlier
AirPort Admin Utility for Graphite and Snow routers Version 4.2.5
Adobe Director MX 2004 Version 10.2