any good shared calendar application?

I have a client who needs to have a calendar that they can utilize and also have their secretary utilize for appointment making. The only other requirement is that they are able to sync with their blackberry, so I assume it needs to be some sort of calendar standard (unless BB only syncs with outlook??, I don't have one so I am not sure.)

So the problem is they can not afford a small business server right now, which would make everything super easy using Outlook and Exchange. They just use a mix of Outlook 2007 and Outlook Express (Win XP), with POP3/SMTP accounts.

I am looking for a piece of software for calendar management that would offer these shared capabilities. They do have things networked, and they do have a central share location for a data store.

I know there are services like google calendar, however I think they would like to keep the data in house as it is a small law firm, so there are confidentiality issues there.

Any thoughts on software that could handle this?

I would have suggested a CalDAV server, but it looks like blackberry devices don't support CalDAV. Zimbra appears to support blackberry devices and it integrates into Microsoft Outlook. Since Zimbra doesn't support OutlookExpress (not that I could see), I would move those people off OutlookExpress and onto Thunderbird.