You know what? Calendar is back!!!
Remember how handy was the calendar in Netscape Communicator 4?
You can now install the Lightning calendar as a SeaMonkey plugin: https://addons.mozilla.org/fr/seamonkey/addon/2313
Unfortunately, it works on 32-bit systems only.
Don't cry, a kind contributor compiled a 64-bit version: http://releases.mozilla.org/pub/mozilla.org/calendar/lightning/releases/1.0b1/contrib/linux-x86_64/lightning.xpi
Where I worked before, we used the Netscape calendar to share plannings across employees. And I still do it with my girlfriend (using Sunbird but as we both use SeaMonkey, I guess we'll go back to Lightning).
Sharing a calendar is easy! All you need is an FTP server.
Just create a new calendar, export it as "iCalendar (*.ics)" and upload the created ics file to the FTP.
Now in Lightning, add a new network calendar and put "ftp://login:firstname.lastname@example.org/calendar.ics" as location. That's it!
I grant you this is not secure, but it's really handy!!!