Calendaring (server side)?

[ken at flyingtoasters.net]

Hi, all.  It's come to my attention that the only real reason I can't just
outright dump Outlook on my end-users' machines is because of calendaring.
 Furthermore -- regardless of platform -- it would be super-duper handy to
have a server-side scheduling/calendaring application so's I could (say)
schedule conference rooms, see that "Bob" was in Atlanta on Tuesday, etc. 
A quick perusal of Freshmeat turns up several contenders...

most of which stopped active development in 2000 or 2001.  A couple seem
to be extant, but either in alpha, or with virtually no docs.

So: I'm looking for something, server-side, that's RFC 2445 (iCal)
compliant, that someone's actually -used-, preferably with multiple
RFC-2445 clients (eg. Outlook, Mozilla).  OSS would be preferable, but I'm
willing to pay if need be -- though I'd -vastly- prefer a Linux-based
solution over a Windows-based one.

Any suggestions?

Thanks,

-Ken