You probably heard about GIMP, the free photoshop-like software. This is an open source project, which resulted in a robust software with many features, you may even compare it with Photoshop, though Photoshop was on the market for years and they have a lot of money and time to tune up and upgrade the application.
What is GIMPShop? Well, GIMPshop is a adjustred version of the GIMP. The idea was to make the UI more like Photoshop. And now GIMPshopw is available for Mac OS as well as Windows. You may want to findout the difference between Photoshop and GIMPshop, so read the post at blog on GIMPShop.