Run is one of those games that is extremely simple and ridiculously addictive. Basically, you have to run and jump your way through an obstacle course. You run at a steady pace, but each time you complete a level that pace gets a little bit faster.
The game takes on a whole new level of complexity when you realize that in addition to moving left or right or jumping to avoid a hole you can also walk up the walls to your right or left. By the time you get to level 10 or so, this technique becomes essential to your survival.
Another interesting feature to this game is the ability to create your own levels through the use of an extremely simple and user-friendly level editor.
The game's not particularly hard (at first!). You never seem to run out of lives. But it's the kind of game that's hard to stop playing once you start. http://www.kongregate.com/games/player_03/run?acomplete=run