These are full-featured open source software products, free as in beer and speech that I started to use recently. Vivek Gite picks his best open source software of 2013.
#1 Replicant - Fully free Android distribution
Replicant is entirely free and open source distributions of Android on several devices including both phones and tablets. I have installed it on an older Nexus S. You can install apps from F-Droid store a GPLv2 client app that comes configured with a repository hosting only free as in freedom applications.