Halo may never appear in 2d officially, but in its absence some enterprising Frenchmen have come up with a freebie version instead. Based on the Halo novel The Fall of Reach, Halo Zero is a 21MB download.
It plays a bit like ageing PC shooter Abuse - you control the Master Chief, for it is he, using the cursor keys (and space to jump), and use the mouse to point either side of him where to want to fire and the left mouse button to deal some death on the little gnome people and their big blue friends.
There are quite a few levels, and although we only had time to make sure the thing worked before getting on with telling you about it, we can report that it looks surprisingly nice for a completely fan-made effort. Sort of like a LucasArts adventure, maybe.
You can take a closer look at Halo Zero on the game's website. Just don't tell Microsoft or Bungie.