All of the images here were created using Halo 3's saved games and screenshot features. And a little Photoshop. I'm certainly not the first to create panoramic images this way, but I do want to detail the process in case you're interested in making your own. The process stems from a long-standing photographic technique of using multiple images sewn together to create a larger whole.

Photoshop is the standard in image-editing, so it is the tool of choice for this tutorial. I am using
Photoshop CS 3, but the cheaper Photoshop Elements also allows for automated image-stitching.

There are other tools you can use besides Photoshop to stitch images together. Here are a few:
Hugin - free!
Realviz
Panorama Tools
Autopano
The Panorama Factory
Step 1:
Find a scene that you want to take a panoramic shot of. This could be scenery, or it could be an action shot, or whatever you like.

Step 2:
Experiment with viewing angles. Try to find an angle that allows you to rotate the camera and see the entire scene. You want to find a place from which you can lock down the camera and not have to move it again in 3D space. Look for an angle that gives drama to the shot as well. Composition is key.
Step 3:
Your shot can be either horizontal or vertical, and the technique is the same for both. Take a series of screenshots by starting at one extremity of the shot and moving to the other one. Be sure to "overlap" the images. In the photographic world, the overlap is usually around 40%. I find this ballpark value works well for Halo 3's camera as well.
It's dependent on the focal length of the lens, and the zoomed-out H3 "lens" is on the wide end of normal. You want to "overlap" so that later on you'll have plenty of common elements to help blend the images together. DO NOT move the camera with the left stick! Rotate it only using the right stick. You may be okay if you budge the camera a tiny bit, but it's easier to just lock it down.
Step 4:
Download the images from your profile on Bungie.net and save them to your computer. Make sure you get the high res versions.
Step 5:
Open Photoshop. In my case, I'm using Photoshop CS3. You can use any image-editing tool you like, but Photoshop CS (particularly CS 3) has a killer tool called Photomerge. No need to do these manually. Such would be the acme of tedium when a piece of software is more than willing to do it for you.
Step 6:
Run Photomerge! To do this in Photoshop, go to File > Automate > Photomerge. You'll then be asked to locate the files you wish to use for the process. CS 3's photomerge also has several options for the way in which the images are to be stitched, depending on what kind of "projection" you want. I have had great success using the Auto option. Leave it on Auto, load in the screenshots you retrieved from Bungie.net, and let her rip.
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Step 7:
Photoshop will now begin to do things on its own. Let it work. The time it takes to assemble the final image depends on all sorts of things, like how many images you used, how strong your computer is, and whether the images have easily-recognizable features to simplify the match-up. If your image has a lot of blank sand from Sandtrap or snow from Snowbound, for example, the resulting image may not be to your liking. There is very little contrast in the sand and snow, and the software doesn't quite know how to match it up. Make sure you have strong contrasty elements, like people, vehicles, weapons, architecture, or anything that gives the software reliable pixels to compare.

Step 8:
If all goes well Photoshop will create a file that has each screenshot as its own layer, masked off and everything. Flatten the image. You don't need the layers.
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Step 9:
Crop the image. It should be on a transparent background, and distorted into a non-rectangular shape. This distortion is somewhat unavoidable, but can be partially corrected using the Lens Correction filter. However, correcting the distortion usually results in loss of resolution. Eventually, You'll need to select the portion of the image you want to keep, and discard the rest by cropping it.
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Step 10:
Your call. If you're a purist and don't want to mess with the way the game spits out color and depth, leave it be. Personally, I find that the colors in a normal screenshot straight out of the game are flat. I say "normal" screenshot because the game creates some pretty awesome effects if you shoot from inside an explosion. Those screenshots don't need much adjusting, IMHO. But I don't see anything wrong with adjusting Levels, Curves, or colors. You've already gone and made an unnatural screenshot anyway by the mere act of stitching them together. Might as well go all the way and add some subtle punch. Or not so subtle. Like I said, your call.
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