Here is my favorite list of Outlook add-ins:
OfficeCalendar (http://www.officecalendar.com) - lets you share Microsoft Outlook calendar, contact and task information without Exchange Server.
Extended Reminder(http://www.slovaktech.com/extendedreminders.htm) - enables reminders on items in any email, contacts, tasks or calendar folder in your default mail store except the Outbox and Deleted Items folders.
Anagram (http://www.getanagram.com) - capture information from emails, web pages, etc. and creates Outlook contacts without having to retype them.