Sidescroller

Alien Swarm is a free game and Source SDK release from a group of talented designers at Valve who were hired from the Mod community. Available free of charge, the game thrusts players into an epic bug hunt featuring a unique blend of co-op play and squad-level tactics. With your friends, form a squad of four distinct IAF Marine classes.

Shank is a 2D side-scrolling beat 'em up developed by Klei Entertainment. The game features both melee and ranged combat, as well as some acrobatic gameplay. The story was written by God of War co-creator Marianne Krawczyk and tells the tale of Shank's quest for revenge.

Platform/puzzle game by Frozenbyte. Reminds a lot of The Lost Vikings.

Developed by a two-man team, Aquaria is a sidescrolling action/puzzle game. Although the gameplay is two-dimensional it is really a 2.5D game (as opposed to 2D), using animated and skinned polygon models. Help lonesome Naija find her way through underwater worlds and unravel the mysteries of her very existence. Learn siren songs to cast various spells or turn into an energy bolt-firing warrior form. Swim your way through reefs and abysses where beautiful or deadly creatures dwell and long-forgotten secrets await.

A modern re-imaging of the classic NES platformer. A funny little title with a retro feel.

Wiggles in Germany, Gnomy in Russia. Apparently knows as Diggles somewhere else.

Duke Nukem: Manhattan Project is a side-scrolling run and gun computer game developed by Sunstorm Interactive.

Switchfire is a classic side-scrolling shooter. Main features of Switchfire include three different vehicles (Jeep, Helicopter and a Ship) and 15 action packed levels. The game also supports 2 player co-op.

One player controls a helicopter and other a jeep. The jeep can transform into a boat when water appears on screen. Players can upgrade their weapons with grenades and missiles and more. At the end of each level there is a "Boss", one last tough and big enemy before your mission is complete.