I currently use Workflowy. It is not FOSS but it is has some nice features. It syncs between devices, has a decent mobile client, works offline, seems to have

I currently use Workflowy. It is not FOSS but it is has some nice features. It syncs between devices, has a decent mobile client, works offline, seems to have a good developer, doesn't cost an unreasonable amount, has pretty good multi-user features.