the tk extension extracted from core and gem-ified
gem install tk_as_gem

originally extracted from 1.9.1p376, then added a minor change to make it work with mingw/msys.

To install into an existing mingw distro:
install ActiveTcl [c:\tcl] (should add itself to the path)
C:>gem install tk_as_gem

Note: ruby.exe and your tcl dir must be on the same drive.

Might install for linux, as well.

Note that this only works for 1.9 -- for 1.8 mingw you can install some binaries from here:
http://github.com/rdp/ruby_windows_tk

Bugs:

currently to exit out of ruby, once you've done a
require 'tk'
of the gem, you must *first* close all outstanding open Tk windows, THEN exit your ruby process, otherwise it will hang.

Also you "might" need Tcl/Tk 8.4 instead of 8.5 (you certaintly do, for 1.8)