These patches implement various features for Open WebMail 2.10, and are sponsored by Moatware, a firewall manufacturer incorporating Open WebMail into some of its firewall units.
Open WebMail is available under the GNU General Public License; so are these patches.
With this feature, you can allow other users on an Open WebMail installation to view your calendar. You can also allow them to add entries (and edit/delete entries they created), or give them full access. You can mark calendar entries as private if necessary.
This is still a work in progress, of course. Check here occasionally for updates.
Download and apply owm-2.10-sharedcal.patch:
Use cd to change to the top-level directory of the Open WebMail source code distribution, i.e., the directory that contains the cgi-bin/ and data/ subdirectories.
Example: on a system where you have /var/www/cgi-bin/openwebmail and /var/www/data/openwebmail directories, you need to do "cd /var/www".
Run: patch -p0 < /path/to/owm-2.10-sharedcal.patch
Also, you will need to download sharedcal.gif and place it in the data/openwebmail/images/iconsets/Default directory. It's ugly, but I'm not a graphic artist. I haven't made images for the other iconsets yet, sorry.
You might want to read these notes. They provide gory implementation details explaining why the patches aren't perfect. They seem to work just fine for us.
If you ever plan on putting these into production, your calendars won't work with stock OWM 2.10 or earlier.
When you go to your calendar, you should see a "Shared Calendars" icon that will take you to the Shared Calendars panel. You can add other usernames on the system (you may have to type in full email addresses on installations supporting multiple domains) and select their access level. You can also view the calendars of any user that has chosen to allow you to do so (a pull-down menu lists those users).
Screenshots are on a separate page.