Maemo - maintained by the community
Moblin - dead (development stopped by Intel)
MeeGo - dead (dumped by all participants after Nokias 11/2 Elopocalipse)
Meltemi - dead (development stopped by Nokia)
Tizen - status uncertain (developed behind closed doors, which also causes issues like the one described in this article)
Mer - alive and kicking (=
MEeGo
Reconstructed - continues where MeeGo originally stopped; it is basically a metadistribution without any GUI; can be easily used for building fully functional mobile distros)
Jolla - alive (based on mer with custom UI & polishing; should get its own device/s soon)
Sailfish - alive (this is basically just Jolla licensed to other OEMs; basically like Iceweasel in Debian)
WebOS - alive but uncertain (opensourced, but loosing support for running on its original hardware in the process; currently no plans for releasing further WebOS devices)
Bada - dead (development stopped by Samsung; parts might land in Tizen to universal acclaim

)