Linux only (maybe): 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.
Image via Open Source.
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.
The main Google Code site for Gimpbox is Chinese language; Google Translate does a fair enough job of bringing the message over to English. A full set of instructions and explanation, however, can be had at the OMG! Ubuntu! blog. Gimpbox is a free download that work on Linux, but could possibly work on Macs or Windows too, through some Python tweaking.
GIMPBox gives stable versions of The GIMP single window mode [OMG! Ubuntu!]