This open source PHP script will choose from two different gamma-corrected images based on whether the page is being viewed from a Windows/*nix machine or a Mac. Macs generally have a target gamma of 1.8 while Windows and non-Apple *nix has a target of 2.2 or 2.4 (which usually nets out to about 2.2 in average lighting). The result is that images made on Macs look too dark on everything else and images made on everything else look too light on Macs. This script solves the problem by detecting which platform the browser is running on and automatically redirects the request to an appropriate gamma-corrected copy that you’ve prepared ahead of time.
This is free and open source software and is licensed under the GNU General Public License.
More info and download here in the Tools & Reference section.