"Linux advocates are familiar with the refrain that would-be
switchers in the graphic arts have to rely on Adobe Photoshop under
Windows because it can do things that the GIMP can't. An important
but altogether different hurdle is the installed (and paid-for)
base of often expensive third-party Photoshop plugins. But a
solution to that problem might be easier than you think.
"The key is a piece of software called pspi (for Photoshop
Plugin Interface), written by GIMP hacker Tor Lillqvist. It is a
GIMP plugin that acts as a bridge between the GIMP and Photoshop
plugins; to the Photoshop plugin it looks like a full, running copy
of Photoshop. It provides the hooks into the menus and functions of
Photoshop that the plugin expects to see, and connects them to the
GIMP's extension and menu system..."