My usual solution for this is C++/Qt. Runs well on Linux, and on Windows you can statically link Qt into the single .exe if your small app is FOSS (at least LGPL-compatible), otherwise you should ship a few extra .dll files.

Compared to Tcl/tk you get (A) catch errors at compile time, and (B) Qt apps usually look better than Tk apps.

With mxe+qmake it's one command to cross-compile from Linux to a Windows binary.

I am now looking at the Red programming language [1] for exactly this, but it does not have the libraries of C++ or Tcl. Still it is under 1 meg complete with libraries and it is homoiconic, cross-platform, and has a REPL and a systems language Red/System for going low level. The GUI and UI DSL and drawing DSL work on Windows and Mac for now, with Linux close behind.

  [1]  http://www.red-lang.org/p/about.html

There's also Lazarus in this category.

The default download didn't have GUI support on my Mac when I just tried it, and the documentation seems to indicate it's Windows only so far?

@mappu above (no reply link by his reply):

I like Lazarus a lot. I am not a fan of Pascal though, and Red is under 1 MB batteries included. I like how it seems a cross between Lisp, Forth and other languages.

I think it is in 0.63 or the latest binary now available. I'll check on my iMac later and get back to you. I use it on my Windows box for now.

Back in the day a big mistake from Tk devs was how it just looked like Motif everywhere.

Eventually that got fixed, but we were already busy with other languages.