There has been some interest in authentication plugins for mosquitto recently. Some examples have appeared:
Authentication based on md5 hashes: mosquitto_auth_plugin_md5
Authentication based on md5 hashed passwords in postgresql: mosquitto_auth_plugin_pg_md5
Authentication and topic ACL with redis and a PBKDF2 hash: mosquitto-redis-auth
I particularly like the redis based plugin for the interesting additions like the "superuser" that is exempt from ACL checks.
If you've written an auth plugin and think it might be useful to others, let me know.