"The free, open-source image editor, GIMP, uses multiple windows
for its tools and dialogs, but they're getting combined into a
single window in a future release. Impatient types can combine
everything right now, though, with a Python script.
"Gimpbox is an alternative launching point for the GIMP editor
that locks the toolbox panel, layers, and brushes onto the main
window, leaving other tools to launch in separate windows.
Modifying the main GIMP to accept Gimpbox's remix requires just a
small bit of command line work, and making Gimpbox the default view
for your editing is accomplished with a shortcut edit."