Line Rider is a 2D game without any real goals, it's basically a sandbox game where you are drawing lines for a little guy to ride.
There are 3 differents kind of lines (colors are only visible in the editor, in game everything is black & white):
-the normal lines, they are blue and your guy will ride on them
-the accelerating lines, they are red and will make him accelerate.
-the scenery lines, they are green and used for scenery
You can even have him to do wheelies, go through loops,... When your track is ready, you can start it and see if it's done right or if this guy crashes.
Here is a Linerider video