Here is the link to SPSource on codeplex. Basically it allows you create your sites/lists/content types/etc in the SharePoint UI and export it all out as CAML. This is a big saver (if your using CAML for deployment) for site creation/deployment.
It is a command line tool that takes one input parameter, the url to your site. It will create the xml files for you.
This is much easier than hand coding the CAML for your site.
Where I see this adding tremendous value is where you have a customer who you have deployed a SharePoint site for. They make changes through the UI (add navigation/sites/etc) and now your code base and the prod site are out of sync.
You can use SPSource to pull down what it takes to create their modified site and merge it with your code.