Qt works just fine on *nix, Mac, and Windows. GTK+ isn't quite so nicely polished, but is still viable on all three. Installed base is irrelevant if you includ

Qt works just fine on *nix, Mac, and Windows. GTK+ isn't quite so nicely polished, but is still viable on all three. Installed base is irrelevant if you include the libraries or statically link. You'll bloat your download a few MB, but it's manageable.