"BTW, you can download it and test it out for yourself. It's available for MAC, Windows, Linux and any other POSIX-like environment that you like."
I'm very well aware of what the GIMP can do. I worked on Photoshop for many years and always kept an eye on what the GIMP did. For OSS, the GIMP's features are nice and acceptable for some work. As a free alternative, it's great if you think it does what you need. Especially if you can handle the UI, which I find pretty much a deal breaker. Typical of too much OSS, IMHO the GIMP suffers from a very poor UI, even after version 2.
But the GIMP does not have a feature that does what Shadow/Highlight does from what I have seen or can tell. Just experimenting with the Filter Pack you point out shows that it doesn't do what Shadow/Highlight does.
For example, in that Filter Pack dialog, how do I adjust the shadows and the highlights *simultaneously* to correct in one pass with the algorithm analyzing the adjustments as they relate to each and not go two passes which creates a more degraded result?
Better yet, why don't you post a sample or have Dunstan send you the original image and prove the results "are the same."
"I, however, cannot download, purchase, or steal a copy of Photoshop to test out on my Linux workstation."
Given that, maybe you should reconsider espousing an opinion based on limited knowledge -- that being a lack of taking the time to grab a Win download of Photoshop and test it out on a Win partition on your Linux box.
Since you have not used or tried Photoshop since 1998 and are unwilling to find a way to see what it does do, maybe you should reconsider a statement like, "Now I know how well Gimp performs with respect to Photoshop. I'm happy to confirm that in Gimp 2.0 I get identical results."
You are speaking from an admitted lack of knowledge -- as if to encourage others to believe the same thing -- forming an opinion on half of the facts. So you withdraw the "identical" portion of the statement... I suspect with a little more investigation, you'll discover it's even more than that.