Codename: Gordon

Codename: Gordon is a 2D flash-based side-scrolling shoot 'em up developed by Nuclear Vision for Valve Software.

In Codename: Gordon players undertake the role of Gordon Freeman, the very same mute character from the immensely popular and critically acclaimed Half-Life series, and lead him through various locations inspired by the Half-Life universe, including City 17 itself. The game features several recognizable enemies from Half-Life including the headcrabs, zombies and the Combine, and also features a few of the main characters such as Alyx Vance. Gordon is armed with various weapons, such as the trademark crowbar and the gravity gun, introduced in Half-Life 2. You also communicate to other characters through text boxes by using emoticons, such as :] and :-), to choose your answers.
 
Upon finishing the game, a new bonus game is unlocked, called "Crow Chase", in which the player has to try and gather as many points as possible within a given time limit by chasing crows, in an attempt to keep them in the air for as long as possible. 

Codename: Gordon was exclusively distributed through Steam and not available to buy elsewhere. You can install the game by putting steam://install/92 in your address bar.
 

Plot

 
The story of Codename: Gordon presents an alternate version of the story in Half-Life 2 with some locations being inspired by both Half-Life and Half-Life 2. Through his adventures, Gordon Freeman meets up with some of the other characters from Half-Life 2 to find out the cause of the disappearance of the third dimension. Through his adventure, he meets up with Barney, Alyx, Eli, Dr. Kleiner and the G-Man. Gordon eventually finds out that a strider, a large tripod war machine, turns out to be the source of the problem. Upon defeating it a portal is opened in which Gordon steps in, sending him back to the third dimension.