Early Stuff
Xerox STAR demonstration (there's also a much longer recording of a talk that goes into some of the backstory behind the STAR interface and its hardware here)
Rethinking the Desktop
Novel Interaction Techniques for Overlapping Windows
Bumptop
Pen Computing
Teddy
Pen + Touch = New Tools
(In general, Ken Hinckley's page is a great resource for interaction techniques, particularly with pens.)
Multitouch
LiquidText
Neat Layout Gestures
Getting Away from the Desktiop
Digital Desk
Head Tracking for Desktop VR Displays Using the WiiMote
New Interaction Technologies
TeslaTouch