A few days ago I decided to try making something new. What I decided upon was "Sonar".
The basic idea of the game, is that you play as a submarine, deep under the ocean -- Your only means of seeing the world around you is (aptly) 'sonar'.
When I've managed to fully implement the networking library I'm playing with - I plan to add multiplayer capability, and all the hunt-down-the-enemy stuff as well!
In the meantime, if anyone wants to give this a look, be aware it's an early build (only actually put about 5-6 hours in over the last three days.) but a copy is available here:
(Currently only a windows build, if anyone wants something else, drop a message here and I'll see what I can do!)
(Edit:) And before I forget: W/A/S/D or Arrows to move, Left click mouse to send out a sonar pulse, Right click mouse to drop a 'sonar mine'!