I'll get us started with the 2 I know.... If there are others please share and I will add them!
Also if you know of a link with more current information on a rom listed please let me know.
Firefox OS - http://forum.xda-developers.com/goog...rhead-t2601556
Ubuntu touch -http://forum.xda-developers.com/google-nexus-5/general/port-ubuntu-touch-nexus-5-t2594874