I really like the simplicity of zenphoto, but thought a multi-user version would be very nice to have. I looked around but couldn't find much by way of a multi-user version, so I modified the code a bit to support multiple users. You can download a tarball at http://www.creativejoos.com/ and see a working version at http://www.movemberdiaries.com/ (a little test bed site I set up).
Some of the fundamental changes:
I think that's all, off the top of my head.
I did it quite quickly, and while I think I understand how user authentication and general security works, I can't vouch that all my work is hole proof, so use at your own risk.
If it's useful let me know :-)
My bad, should be sorted now. Also, not sure if I just forgot to put the full path to the tarball, or if it got chopped, but here it is again. Re-tarballed it after the changes.
http://www.creativejoos.com/code/multi-user-zen-1.0.8.2.tar.gz