Vim
autocmd BufWritePre * :%s/\s\+$//e
Emacs
(add-hook 'before-save-hook 'delete-trailing-whitespace)
Textmate http://blogobaggins.com/2009/03/31/waging-war-on-whitespace.html
In your existing codebase
find . -name '*.rb' | xargs perl -pi -e 's/ +$//'