The closest I've found so far are a couple from Topaz Labs. They're not quite the same though.
Retinex is awesome. Especially if the environments in which you shoot lead to lots of underexposures.
tl;dr: Sorry, there is currently no exact Photoshop equivalent to GIMPs Retinex filter, though there are a few Photoshop plugins that claim to produce similar results.
For those who may not know what Retinex is, or how it's different from other processing algorithms (I didn't), you can find a full treatise on the technology and it's applications from the technology's inventors here: Processing Digital Imagery To Enhance Perceptions of Realism
To briefly summarize what is admittedly a rather dense white paper, Retinex, or more correctly Multi-Scale Retinex with Color Restoration (MSRCR) is an algorithm that automatically "improves the perception of visual realism and the quantity and quality of perceived information in a digitized image." (whew) In layman's terms, The algorithm at the heart of Retinex is able to take an improperly exposed photograph and enhance shadow and highlight detail in a natural-looking way and in a largely automatic fashion. The inventors of the algorithm claim that they are able to achieve consistently superior results- in terms of both quality and speed of processing- to other image manipulation algorithms on the market.
The algorithm was developed with a grant from NASA, and is a patented technology which was marketed by the TruView Imaging Company under the product name 'PhotoFlair,' both as a standalone tool, a Photoshop plugin, and an Adobe Premiere plugin. I say 'was,' because for reasons I couldn't dig up in my searching the company abruptly ceased operation, distribution and technical support of its product line sometime in 2003. There is no indication, however, that TruView Imaging Company has released the patent on the algorithm, which leads me to surmise that the Gimp Retinex plugin is an approximation of the MSRCR algorithm, not the 'real thing.' Additionally, I can find no indication that anyone has made port of the GIMP plugin for use in Photoshop, thus the simple answer is that there is no exact Photoshop equivalent to the GIMP Retinex plugin.
There are, however, a few different Photoshop plugins who claim to have surpassed the enhancement capabilities of Retinex, most notably is the 'Retina' Photoshop plugin by Xtra Sens Software. As I have not used the product you would want to download their 30-day trial for further comparisons between the products.