With Snow Leopard in users' hands for less than 24 hours, reports of software incompatibilities are coming in (as well as what's been updated). Apple has posted a list of its own to the knowledge base. The article notes that, when Snow Leopard is installed, known-incompatible software is moved to a folder aptly named Incompatible Software. Additionally, known-incompatible software is prevented from opening (you'll see an "Incompatible software" message).
As of this writing, there are 21 pieces of software on Apple's official list, including Parallels Desktop v. 2.5 (and earlier) and AirPort Admin Utility for Graphite and Snow v. 4.2.5. A few of our readers shared their experiences in our liveblog earlier tonight, including installation troubles. We're sure that all vendors are working to update their products. If something continues to give you trouble, hold tight. Help is (probably) on the way.