Apple usually gets it right with OS releases but there's no denying that Snow Leopard has been blighted by more program compatibility problems than most.
If you want to know whether a program installed on your machine is compatible with Snow Leopard, SnowChecker allows you to do so. It works simply by checking your programs against the extensive list of apps that work and don't work with Snow Leopard at snowleopard.wikidot.com.
SnowChecker obviously isn't much use to you if you've already purchased a program and found out it's incompatible. However, if you're still using Leopard and aren't sure whether to upgrade, SnowChecker could save you a lot of tears. It does allow you to download updates from the developers to make program compatible.
If you haven't already upgraded to Snow Leopard and are worried about compatibility problems, SnowChecker is well worth trying.