Codename: Gordon

"Codename: Gordon, also known as Half-Life 2D, is a two-dimensional Flash sidescroller shooter game made by Paul "X-Tender" Kamma and Sönke "Warbeast" Seidel. The game was produced on behalf of Nuclear Vision, and was distributed over Valve Software's Steam online delivery system as a promotional title for the then-upcoming Half-Life 2. The game has since been removed from Steam's storefront due to the developer's bankruptcy." Source: Wikipedia, "Codename: Gordon", available under the CC-BY-SA License.