Blackberry Enterprise Server.
With BES you can tie into your Exchange server at your office and forward emails to your Blackberry handheld.  Your entire Outlook can synchronize to your phone wirelessly!  Contacts, calendar, notes, and tasks can be synced in real time.  Imagine you are out of the office and your assistant receives a phone call from a client requesting to change the time of their meeting.  Your assistant changes your calendar to reflect the new time and adds a reminder to let you know that it changed.  Within seconds your phone rings giving you a reminder and shows up on your handheld calendar.  Someone in your firm wants to have a company meeting and invite you but you will be out of the office for the rest of the day and he needs to know before you go home for the day if you can make it.  He can create an appointment in his calendar on the Exchange server and put you down as an invite.  You receive the meeting request via email on your phone asking if you want to accept, decline, or request another time.  You accept the time and it adds it to your phone calendar and your calendar at your office while doing this all from your phone.