I have an old Kodak Easyshare. For metadata it encodes camera type and almost all the camera settings (shutter speed, aperture, and so on), zoom setting, and camera body serial number, plus other less obvious stuff.
Some “modern” cameras also encode GPS data, though if you are shooting in a shielded building or under heavy overcast, the last GPS reading will be used.
Not all “old” cameras use serial numbers in metadata.
http://www.amok.am/en/freeware/amok_exif_sorter/ is one program that sorts, http://www.wisegeek.com/how-can-i-view-the-exif-data-of-a-photograph.htm is a viewer online, http://support.google.com/picasa/bin/answer.py?hl=en&answer=19612 is another.
It’s a good idea to check what you post online in pictures! I also routinely remove license plates, clothing emblems, and the like that gives traceability to the picture. Wouldn’t do to show of my Barrett or my M60 with my license plate visible on my truck!
I find metadata useful when doing technical photography, where I might want a folder of all the shots I took at ASA400, 1/60th, and f11. I can sort pix by metadata, with a tool I got online that sorts by exif data (which is basically what metadata is).
Thanks for the warning, DB. I also have some very choice firearms, including a Barrett, and never have posted any of my personal Class III units online, either. Just mfgrs’ pics of them, glass, mags etc, but not my actuals.