It is very hard to find realtime shots, using google images.
...
However, all pre-rendered Halo3 artwork seems to have excellent HDR applied.
assen did a great job showing me the HDR in the Halo3 pr-shots. Could he, or somebody else find a realtime, ingame screenshot that also has it? I gave up after 5 minutes
Err... a few seconds on youtube resulted in many hits:
Here's one that shows many examples (albeit the light sources are mostly player placed objects like those glowing grenades and glowing force fields, etc.):
http://www.youtube.com/watch?v=VLyiaHaxF4w&feature=PlayList&p=nPpoxCikRSs
Just pause the video to examine a still if you need to see it without motion.