For the uninitiated, Bethesda always releases the toolset they use to build their open and dynamic game worlds. This allows for an infinite amount of free, user-generated content to be distributed as mods for their titles.
As far as this... "surprise" goes, I don't think it's going to be a TF2 hat. My guess... and this is a huge, juicy "what if"...
Theater Mode. Yes, a la Halo 3.
HOW. SWEET. WOULD THAT BE.
Dovahkiin, let me know your thoughts and speculations on this Creation Kit present. Alternatively, what's the first mod you'd make in the Creation Kit?