Here's one for the "duh!" category:
I live in Outlook. I always have a problem staying focused on what I'm working on, and avoiding new email. For example, when a new message comes in, I tend to stop what I'm doing, read the email, then decide how to process the email, then I have to get back in the groove of what I was working on previously.
Well, Outlook 2003 has a cool new feature where you can easily transition from working offline or online without having to restart Outlook. So, what this means is that I can get into Outlook first thing in the morning, process new messages and arrange my tasks for the day, then switch to offline mode and get to work. This way I'm not interrupted by new emails. I'll switch back to online mode before and after lunch to process whatever may have come in, then go back offline for the remainder of the afternoon until about 4PM and do my afternoon wrap-up.
It may seem relatively minor, but it's amazing how much more focused I am on the task at hand when I'm not bothered my new email.