Can a user create and manage shared calendars exclusively from Outlook Web Access?
EDIT: We're working with Exchange 2010 only
To the best of my knowledge, there is no simple way to do this. You can manage other users' calendars by simply browsing to http://your_mail_server/Exchange/username/calendar/ after logging into OWA, but as far as managing a calendar not directly assigned to a user that is shared... I'm completely unsure. Exchange 2010 actually has this functionality built-into OWA, and you can find the "open shared calendar" under the share-button.
This functionality is also somewhat exchange-version specific:
Exchange 2003: http://support.microsoft.com/kb/821900