ToNToN CoPT has posted an excellent collection of Photoshop scripts that make aging photos a breeze. There are other packages that do similar work, but his approach replicates the look of photos based on the era they were taken.
This collection is so useful because photographs are easily the most cost effective props available for live action and tabletop games. Prints are ten cents or less at almost every drugstore or retailer and they're inherently immersive. Frankly, I'm surprised they aren't more widely used in commercial products. The source files for a couple of retouched photos can add immeasurably to the immersiveness of a scenario while costing just pennies to make available.