rockdodger is a simple 2d side scrolling game written by Paul Holt. I can't believe how much fun I've had with this game.
http://code.google.com/p/rockdodger/ I didn't like version 0.6 so much.
I recommend version 0.4.2, which you can download here:
Here's a great patch by Josh Grams for rockdodger 0.4.2 that makes it display your score formatted nicely in minutes, seconds and tenths: http://jasonwoof.com/downloads/rd-0.4.2-josh.patch
To apply the patch, download it,
cd to the directory where you have
unpacked rockdodger (where you will find main.c) and:
$ patch -p1 < path/to/rd-0.4.2-josh.patch
Here's a silly patch which takes away friction (optionally) and changes the engine plumes graphically a bit: