Steam's Mid-Week deal is on this little action indie game called "Plain Sight".
At the moment it's 80% off (normally £7.99), or you can purchase 4 copies for £4.80 (normally £23.99) and gift the others to friends.
Plain Sight is a multiplayer arcade game about suicidal ninja robots. Fly through space, leap over planetoids and destroy opponents with your trusty katana. Killing fellow robots lets you steal their tasty, tasty energy. Packed with spinach-like goodness; energy makes you bigger, stronger, faster and generally more awesome.