Didn't notice earlier— one of my favorite Outlook/Exchange tools is now compatible with Outlook 2010. Outlook Spy is primarily a tool for Outlook/Exchange developers, but Exchange administrators also find it useful. It allows you to look under the hood of mailboxes and messages. Created by Dmitry Streblechenko, an Outlook MVP, Outlook Spy has been on my list of "must have Exchange tools" for as long as I can remember. Released in November 2009, the latest version of Outlook Spy (v2.15) adds Outlook 2010 compatibility.
You can download Outlook Spy 2.15 from Dmitry's web site. Registration for a single user license is $49.99. It's been worth every penny and more for me.
I also like the free MFCMapi tool on Codeplex - Microsoft's open source community site where you'll find a lot of useful tools and apps along with the source code. MFCMapi is a compact executable (760-873K) and doesn't require installation. It was created by Microsoft's Stephen Griffin. It's available in both 32-bit and 64-bit versions.