Art Rage. The best free program I've ever seen and a great example of what a GUI can be. Make sketches, oil painting, charcoal work, felt tip marker work, etc. etc. Great program for a Tablet PC or any pen enabled UPC.
eReader Pro. Best ebook reader in my opinion.
FeedDemon RSS aggregator. Great use of the small screen and the only way to keep up with hundreds of RSS feeds. Well, not the only way but the way I prefer.
Karen's Replicator. Free utility I've used for years for syncing or backing up directory trees. Lightning fast.
OneNote. Simply a great program made better with full inking.
Outlook in landscape orientation.
Outlook in portrait orientation. Notice how it automatically reconfigured the 3 viewing panes when I switched from landscape to portrait.
PersonalBrain. I've just started experimenting with this. Kind of a cross between Mind Mapping and visual file indexing. Is very well implemented and only lacking a way to add emails to the map.
SharpMT. Blogging tool for Movable Type blogs that lets you compose posts both on and offline. Great use of the small screen as each screen component can be expanded / collapsed at will. Free.
Tablet PC PowerToy Snipping Tool. Let's you draw a "circle" around information in ANY application and copy it to another document. Great for demonstrating something in an email. Free.
True Launch Bar. TLB provides a lot of information in the taskbar, allowing full configurability and customization. Perfect for the small screen.
True Launch Bar battery monitor.
True Launch Bar Weather add-in