Very useful little program to view and manipulate EXIF headers in JPEG files. All OS'es (Linux, Windows, Mac, etc.)
Example:
$ jhead "-n%Y-%02m-%02d %02H%02M%02S %f" *
Renames all JPEG files in current direcory to something like:
2005-06-21 091337 oldname.jpg
Also it can rotate file accordingly to camera orientation sensor reading (using jpegtran) and clear rotation flag and remove all "junk" data (comments written by Photoshop, thumbnail, etc.) - very useful!
See comments for scripts using jhead.