Sass is bundled with Haml, so to get it you’ll need to install Haml as well.
Standard Gem
Most people will just want the standard stable Sass gem. To install this:
gem install haml
haml-edge Gem
To get the latest and greatest Sass goodness, though, you’ll need something else. A haml-edge gem is created automatically every time changes are made to the master branch. To install it:
gem install haml-edge
Note that gem "haml-edge" must be used before require 'sass' if haml-edge is to be used.
Installation from Git
Haml can also be installed directly from the Git repository:
git clone git://github.com/nex3/haml.git
cd haml
rake install
Downloadable Archives
Finally, archives of current and old releases are available on Rubyforge.