The UDK is extra exciting to me. Not only is the free release of such a prolific current engine staggering, it's surely more appealing to the lovely modding community. Standalone releases are an obvious benefit and will no doubt help them find audiences, but with access to the complete framework I expect them to produce delicious fruit.
Of course, the UDK could be used for almost anything by almost anyone. We're certain to see some weird and wonderful creations from amateurs and veterans alike.