Security/2 is a multi-user system for OS/2. It provides controlled access to files, drives, drivers and other OS/2 stuff, based on access control list for each user.
Security/2 has modular structure, can be easily (i hope ;)) extended and provides a API for developers to enable their applications be multi-user.
Most of the Security/2 utilities are open sourced, that provides additional help for developers.
Free for personal use.