MS Office:

Better Solutions: a FREE Resource designed to help you improve your skills and knowledge of Office.
Slipstick Systems: Outlook & Exchange Solutions Center. If you use Outlook, this site is indispensable! Utilities, how-to's and other solutions.
OutlookHelp: (a site maintained by  Robert Sparnaaij MS-MVP for Office Systems - Outlook)

Favourite Freeware:

Programs I use and can recommend without hesitation!

All Users:
EditPadLite: a great, free alternative to the lacklustre Windows Notepad.
Irfanview: a great image manipulation program
AM-Deadlink: checks your favourites/bookmarks for dead links and downloads favicons as well. 

Security:
I have many freeware applications listed on the Security page here.

Advanced Users:
EasyCleaner: cleans temp files, Window's registry and other unnecessary junk.
ERUNT/NTREGOPT: Registry Backup and Restore/Optimizer for Windows NT/2000/2003/XP/Vista
Restoration: Restoration is a great freeware 'undeleter'. I have never had the need to use it, but those I have recommended it to have had good results.
Process Explorer: from Technet. Replace Task Manager with this freeware application and really know what's going on under the hood!

Digital Imaging:
Photobucket: Upload Video and Images; great for inserting images in web forum posts.

Worth Paying For:

Acronis True Image Excellent backup and recovery software, I have a review of it here.
Diskeeper 2008: a premier disk defragmenter. Vista compatible.
RoboForm: the top-rated Password Manager and Web Form Filler that completely automates password entering and form filling.
GenuTax: tax software for Canadians.