I really like notjustagrid but I use SASS in my projects. I converted the CSS files to SASS/SCSS so I wouldn't have to do it again.
The original-style SASS files are located in css/sass and the new-style SASS files are in css/scss. To add this to a Rails app, just copy either style of SASS files to public/stylesheets/sass in your application. Also, make sure you have the HAML gem or SASS gem in your Gemfile.
I have tagged branches with v3.0.x and v3.1.x because of the new changes with Rails 3.1. Please use the respective branches otherwise the images won't load correctly.