I would like to take this opportunity to point out a few useful little helpers that have made my life easier on a number of occasions in my day-to-day business:
N2Kview:
Nice tool to display and edit the Outlook NK2 file. This allows you to quickly delete old or incorrect entries from the Outlook autocomplete.
http://www.nirsoft.net/utils/outlook_nk2_autocomplete.html
PFDAVadmin:
Great tool for editing public folders. It has often helped me to correct defective permissions. Even without Outlook, you can quickly edit the public folders and add permissions or replicas, for example. PFDAVadmin works up to Exchange 2007, for Exchange 2010 there is ExFolders
http://msexchangeteam.com/files/12/attachments/entry453398.aspx
Microsoft Remote Connectivity Analyzer:
Not exactly a tool, but still very helpful for troubleshooting when ActiveSync or OWA don't really want to work.
https://www.testexchangeconnectivity.com/
SMTP Explorer:
Small graphical tool for testing the SMTP connection
http://www.outlook-net.de/files/SMTPEXP.zip
JetStress:
Tool for determining the speed of future Exchange installations, can also be used for troubleshooting performance problems:
http://www.microsoft.com/downloads/en/details.aspx?FamilyID=13267027-8120-48ed-931b-29eb0aa52aa6
These were my favorites for now, I will continue the list. Tips are very welcome J