raph, despite its lack of cool-new-thing hipness, the Tk toolkit performs pretty well cross-platform (and people are busy working on making it look more modern on Unix), and has a canvas... And should you so desire, it's not that hard to write new widgets and/or hack on the (very nicely written) C code.

I wrote an app for a client of mine in Tcl/Tk on my PPC Linux machine. The first time we tried it out on his windows machine, it "just ran".