Halo Reach, like Halo 3 before it, has an in-game screenshot tools. Play the game. Watch an interactive playback of what you did. Move the cameras. Snap your shots. In just one day, gamers have taken some amazing shots.
See more for yourself at the Halo Reach section of Bungie.net. Bear in mind, all of these images are in-game. They were not manipulated outside of Reach.