On a regular basis, we at OStatic compile documentation and guidance resources for popular open source platforms and applications. For example, you can find our collection of free, online books on open source tools here, including guides that can get you started with Blender, GIMP and more great open source applications.
One of the best ongoing projects for producing free open source-related documentation is FLOSS Manuals. A while back, the site added an interesting new type of guide to its mix: workbooks. These are a cross between manuals and task-focused guided tours, and FLOSS Manuals has a newly updated one out for the powerful open souce Audacity sound editing and production program. If you're into sound, this is worth a look.