Microsoft designed Exchange Server specifically for sharing calendar, contact and other information, as well as e-mail. See Sharing with Exchange Server Public and Mailbox Folders.
One way for small organizations to share information without installing Exchange Server in-house is to buy space on a server shared with other companies. We've seen prices as low as $10/month per user. You need Internet access, of course, to get to the server. You should also be able to get browser-based access to your mail, as well as access from the Outlook client. Many firms are now offering individual or workgroup accounts on shared Exchange Servers. See Exchange ASP Info.