Cam Fortress doesn't really "do" anything, but it does let you pretend to sneak up on someone on the subway platform with your Spy switchblade drawn, or carry Pyro's flamethrower to a gas station. Developed and launched on Google Play for Android phones, Cam Fortress launched last week, and is free to download.
You may take screenshots of your escapades, but there's no other functionality — though the app's description says you can unlock a "hidden class" if you discover an Easter egg. All nine TF2 classes are featured either in the video or the release notes, so who knows what that is.
Cam Fortress [Google Play]