Several users share simultaneous the same Outlook PST files. Public ShareFolder is a client-server solution for small and medium-sized businesses which makes an Outlook network without Exchange Server possible.
4Team for MS Outlook. Create and share project workspaces in Outlook without server
Microsoft Outlook Add-Ons and Accessories
4Team Corporation UK Ltd: Solutions for Microsoft Outlook, Exchange, Office 365.
OMessenger is an interoffice Instant Messaging Software. Download this Office LAN chat instant Messenger. Best network LAN Messenger