See http://www.slipstick.com/addins/contact_management.htm for some options.

I've played with a couple of smallish Outlook CRM apps that use public
folders, but none was terribly satisfactory. There's MS CRM, but that may be
beyond your budget.

Also, you might post in microsoft.public.exchange.admin to get more "eyes"
on this post - but provide a lot more detail, including your version/SP
level of Exchange, versions of Outlook, business requirements, etc etc etc.