http://2doapp.com uses CalDav to sync task items across iOS, Android and OSX. This works with both iCloud (Reminders) and third-party CalDAV servers, including self-hosted http://sabre.io.

If you sync a 2Do app instance with SabreDAV, you can see that the 2Do developers have added app-specific fields to the standard schema, to store text notes, audio attachments and app-specific metadata, without losing interoperability.

CalDAV clients: http://caldav.calconnect.org/implementations/clients.html

CalDAV libraries: http://caldav.calconnect.org/implementations/librariestools....