The Compass website is at http://compass-style.org. It describes itself as follows:
Compass is an open-source CSS Authoring Framework.
In fact, Compass was the first Sass-based framework. What this boils down to is that by installing Compass alongside Sass we get access to lots and lots of reusable patterns and tools for easily creating CSS. Have you seen the TV show Pimp my ride? If Xzibit took Sass into West Coast Customs, Compass would be the first thing they added!
Put another way, Compass lets you write CSS3 goodies like box-shadow, gradients, columns, and transforms with a single syntax, and it magically creates cross-browser compatible CSS of everything that just works without headaches. It also paves the way for additional plugins to enable incredible, lightweight grid systems that we will be looking at in due course.
The project's GitHub development homepage can be found at https://github.com/chriseppstein/compass.
Note
Note: Compass is charityware. This means that while you can use it as much as you like, you are encouraged to make a donation to help the UMDF find a cure for mitochondrial disease. Once you use Compass and realize how much time it saves, I'd encourage you to make a donation at http://umdf.org/compass/.