There's only one I know of, but it's pretty darn significant - SDL (Simple DirectMedia Layer). This means all applications and games based on it (version 1.2.x or earlier) do NOT work on Snow Leopard - the library doesn't even compile on Snow Leopard either. The 1.3 version of SDL, which is in development, should fix this incompatibility problem, but I don't know when that will be released.