It's been forever since a Halo game was released on the PC. Halo 3, Halo Wars, Halo Reach and Halo 4 have all come and gone, without a PC version in sight. So it's clear that if fans of the series want a game on their personal computers, they're going to have to make it themselves.
Which is exactly what these guys are doing.
Project Contingency is the name given to an all-new Halo game being made using Crytek's CryEngine 3. Over twenty devs, many of them industry veterans, are working on it, and while it's still early days - as you can tell from that video - the scale of the project has me optimistic this won't just be another case of "awesome fan game gets announced then is never finished".
Project Contingency [Site]