eWeek is running an article on groupware for Linux:
Companies looking to migrate from Exchange to a Linux-based messaging system or that want broader groupware functionality will find a good solution in Scalix’s Email and Calendaring Platform. From the user’s perspective, the Outlook experience is very good, although the Web client could use some improvement. Email and Calendaring Platform is priced at $60 per user, $600 for a server with 500 mailboxes and $3,500 for the Enterprise Edition. More information is available at www.scalix.com.
I know that many clients are using Microsoft products only for groupware, so this is important.