Public Folders but you need Exchange to do this.....

Although not part of Outlook, you can start with a free subscription to
Office Live Small Business (http://smallbusiness.officelive.com/).  You
get Live's Workplace along with defining your own web site URL (atop the
officelive.com domain, like <you>.<type>.officelive.com).  You can store
documents online to share with others as you dictate.  There is a
plug-in you can download and install into Outlook to facilitate
accessing and saving your online documents.  An online contact manager
is included as well as an online Calendar (the beta version tries to
emulate Google's online calendar but I like the classic one better).
There's a team workplace for collaboration which includes announcements,
calendar, links, shared documents, tasks, and discussions.  You can
create 100 e-mail accounts within this Office Live SB account.
Microsoft's slant regarding this online collaboration is that you will
continue using their applications on your local host but your files,
discussions, calendaring, and other collaboration features will be
online.  

Google has something similar.  They have the Google Apps service but it
costs money.  There is a less featured free version just by having a
Gmail account where you can have online calendars, dictate which are
shared or private, and online documents along with limited functionality
webapps to edit those documents (so you can edit them anywhere you find
a web browser without having to install local apps).

You'll have to play with each to see which you like or what features you
might use from each.

Hi!
you can try the program MAPILab Groupware Server. Using it, you can create
public folders  for MS Outlook users in a local network with no need of MS
Exchange Server. Here is the product page:
http://mapilab.com/groupware/server/
Hope this helps you

Best regards,
Eugene