For web based options, our company uses WebCalendar:

http://webcalendar.sourceforge.net/

Chronos is more attractive, the demo server is kinda slow:

http://chronoss.sourceforge.net/

Take time testing whatever you recommend.

You have plenty of work migrating them away from Outlook but
it's worth it. For the user, Outlook is a nice program; however, for the 
admin, it has fundamental security flaws, partly because it's so popular
that virus-writers target it. Our company suffered three days of Klez 
virus cleanup (the virus hit us before our antivirus knew it existed), and 
we never used Outlook again.

-Jared