I can think of a couple of ways which are not that easy:
- ftp - user needs ftp (or even ftps) client, active vs. passive, firewalls
- https - only for download
- webDAV - setup of apache is rather extensive
- scp and normal user accounts - hard to secure the system from then local users
I guess I am searching for a simple webDAV server that is easy to setup and already contains sane defaults (and https support).
Should allow for upload-only directories (-wx permissions).