I use a pretty extensive suite of tools as a developer - two IDEs, debuggers, code analysis tools, profilers, etc. But one tool stands out as the glue between all of my tools - OneNote 2007.
Here's a rundown of a few of the tasks where Outlook is invaluable to me:
OneNote's UI seems to work exactly the way I work - I almost never use the menus or toolbars, as it's in the right state already. Just right-click on the right part of my notebook, pick "New Page" or "New Subpage" and start typing. It doesn't interrupt my train of thought with modal dialogs or any other up-front decision-making.
It's also very easy to reorganize content - since it all sits in OneNote, it's easy to drag pages and subpages around, shuffle portions of pages around, etc. When I kept notes with notepad, I had content scattered across many files in many directories and bookkeeping had gotten out of control.
I'm still exploring OneNote. I haven't had a chance yet to look at:
If you're interested in experimenting with OneNote 2007, a trial edition is free to download, from http://us1.trymicrosoftoffice.com/product.aspx?family=onenote&culture=en-US. There is also an online test drive at http://www.microsoft.com/testdrive if your browser supports the Citrix Web Client control and you have a Passport account.