See Replicant: http://www.replicant.us/

Unfortunately it is not well supported yet, as porting it to any given device seems to require rewriting several proprietary Android drivers.

Latest release notes are interesting. Latest supported Android seems to be Samsung Galaxy S3.

Looks like the core maintainer could benefit from donated android devices for Replicant to run on newer devices.

http://blog.replicant.us/2017/02/replicant-6-0-development-u...

Like Replicant? http://www.replicant.us/

As the OP is looking for an alternative to Ubuntu Phone, one has to bear in mind that Ubuntu Phone wasn't 100% free-software either (it used the same proprietary drivers and blobs as Android).

ChromeOS has a dev-mode setting to convert the installation to ChromiumOS.