sdd wrote:shouldn't microsoft have made it open scource by now?
You'll be hard pressed to find any examples of that ever happening.
The closest would probably be id software. Every time they make a new version of their engine they open source the source to the old engine. They don't however open source the game content. The id tech engine makes a very clear distinction between the engine itself and the game content.
This is seen the most clearly with quake 3, id tech 3, openarena and ioquake 3. Quake 3 uses the id tech 3 game engine. Openarena uses the ioquake3 engine. ioquake3 is a fork of the id tech 3 engine and it is so compatible that you can load the quake 3 content on ioquake3 or the openarena content on the id tech 3 engines.
Other than that, it was mentioned that when minecraft sales eventually stop (I think its got tonnes of steam left yet) it might well be released as open source software.
I can't think of any other examples of a company that has ever done what you want.
Technically halo is already abandonware. The mac version recieved the last patch in 2006, other versions even earlier. The 360 re-release is the one they want you to play.
If you desperately want halo on the pi, well, only 1 thing you can do and that would be to clone it. Again, id tech 2 or id tech 3 might be good places to start. But it certainly isnt an easy job to do 1 level even.