The ability to customize toolbars has been around for several versions now, but it doesn't seem to be very widely used in my unofficial survey.

I make heavy use of customized toolbars in Outlook, specifically for adding my own keyboard shortcuts - I generally avoid using the mouse if possible, I'm a big keyboard junkie.

For example:

1. Open Outlook and right click on the Standard toolbar (the one that has the basic buttons like Send/Receive on it)
2. Choose Customize
3. On the Commands tab, drag an item from a menu that you commonly use onto the toolbar. For example, on the Actions menu, drag the Red Flag onto the toolbar next to Send/Receive
4. Right click on the red flag and choose Text Only (Always)
5. Right click on the red flag again and change the Name to &X or another shortcut you'll remember. The & precedes the letter you'll type with Alt in order to access that button - i.e. Alt+X will now set a red flag on selected messages.
6. Drag Actions | Flag Complete next to the red flag and choose a similar short keyboard combination such as &Z.
7. Close out of the customization dialog
8. Now select a few messages, press Alt+X to flag them for followup. I do this to help me remember which items in my inbox need a response from me. Then you can also press Alt+Z to mark the item as complete once you've followed up.