CNET has an excellent article on free Windows utilities at download.com. Featured tools include Firefox, Thunderbird, OpenOffice, Gimp, Paint.NET, MediaMonkey, 7-zip, Foxit, Pidgin, Smart Defrag, WinDirStat and Process Explorer.
Process Explorer is part of the Windows Sysinternals Suite available from Microsoft for power users.
Microsoft also provide free downloads of their Live Essentials applications, which include Messenger, and Writer for offline blog editing. Google Picasa, however, is much better than Windows Live Photo Gallery.
Wakoopa have a utility and community site for tracking member application usage. Application rankings and reviews are available by category.