In this game you are a ship protecting a planet from incoming asteroids. All in all this game took around 24 hours to complete, and comes in at 601 lines of code(including comments.) Izzy545 - Via MSDN Forums

So here is my rather generic 2D vertical scrolling shooter F14 XNA. Think 1942 or Raiden and you'll get the idea.

Particle Wars Game with source.

XNA Express Time Pilot game and source code

XNA Quake project, now the code is available for download to all. This can be a good start for a FPS project. The original Quake level editors work fine for making maps.