I guess most if not all programs we use in Windows has an equal or equivalent in both Linux and Mac. There will always be a handful of proprietary programs that are found lacking in the Linux world. However good The Gimp is it's not Photoshop. Gimp is good enough for what I need it for but I know some people that have found it lacking and will stick to Photoshop.

My only reason for keeping Windows around is my love of FPS gaming.

So if I were to recommend a Windows only program it would be the games that are made for it, like Battlefield:2