RubyGems

RMagic

Install ImageMagick with headers:

http://www.imagemagick.org/script/binary-releases.php#windows 

gem install rmagick --platform=ruby -- --with-opt-lib=c:/Software/ImageMagick/lib --with-opt-include=c:/Software/ImageMagick/include

Disable downloading documentation

In ~/.gemrc:

install: --no-rdoc --no-ri
update:  --no-rdoc --no-ri