In Outlook 2003, you can change properties on multiple items at once. Some examples of when this is useful is to flag or set categories on the items.
1. Select the multiple items in the view
2. Click the flag column in the default view – or – right click and set a category – or – right click and set a different color follow-up flag
OK yeah, this is a pretty weak tip, but I think the ability to do it without right click for flagging is pretty neat.