Here are some examples of interfaces or interaction techniques that left a strong impression on me of just how much room there is for innovation within HCI. Each of these is an elegant combination of simple and familiar elements, and yet is surprisingly novel and/or clever.

• Marking Menus are a scheme for integrating hierarchical menus with gesture recognition. They can be operated in one of two modes: novice and expert. One property of Marking Menus is that the very use of them in "novice mode" will naturally and gradually train the user to operate them in "expert mode", a mode that is qualitatively different and much faster. See, for example, Gord Kurtenbach and Bill Buxton (1994), User Learning and Performance with Marking Menus, Proceedings of ACM CHI '94 Conference on Human Factors in Computing Systems.
• Toolglass allows simultaneous selection of nouns (objects) and verbs (actions, or commands) in a bimanual interface, by clicking through the verb, onto the noun. Eric A. Bier, Maureen C. Stone, Ken Pier, William Buxton, Tony D. DeRose, Toolglass and magic lenses: the see-through interface, Proceedings of ACM SIGGRAPH '93. Gordon Kurtenbach, George Fitzmaurice, Thomas Baudel, Bill Buxton, The design of a GUI paradigm based on tablets, two-hands, and transparency, Prcoeedings of ACM CHI '97.
• The Hotbox is a nice extension of pop-up technology. See Gord Kurtenbach, George Fitzmaurice, Russel Owen, and Thomas Baudel (1999), The Hotbox: efficient access to a large number of menu-items, Proceedings of ACM CHI '99 Conference on Human Factors in Computing Systems.
• Various window managers for X11, such as ctwm, allow windows to be shuffled, moved, and resized very quickly with the mouse, without requiring the acquisition of window borders or other decorations.
• Tape Drawing is a 2-handed technique for drawing curves. Though not invented by HCI researchers, a digital version of tape drawing has been implemented only recently, and demonstrates the surprising qualities of this drawing technique. Curves can be drawn with straight and curved segments, with cusps or with C1-continuous transitions between segments, and with continuous control over the tangency of the curve, all without any mode changes! See Ravin Balakrishnan et al. (1999), Digital Tape Drawing, Proceedings of ACM UIST 1999 Symposium on User Interface Software and Technology.
• John Maeda's concentrics are a fine example of his reactive graphics, which I like to think of as an exploration of the basic elements of interaction.
• Joshua Nimoy's TreeType is a novel reinterpretation of familiar metaphors: typed input, and tree structures.
• Golan Levin's YellowTail demonstrates how both the shape and timing of a mouse stroke can be directly and immediately captured in an animated form. The ideas behind YellowTail seem to overlap with those of John Maeda's TimePaint program (see page 98 of his book Maeda@Media, 2000).
• Venolia and Neustaedter have described an elegant way of combining an outline tree view with a sequential ordering of nodes based on some attribute such as time. See Gina Danielle Venolia and Carman Neustaedter (2003), Understanding sequence and reply relationships within email conversations: a mixed-model visualization, Proceedings of ACM CHI 2003 Conference on Human Factors in Computing Systems.
• Tracking Menus trap the pointer within a kind of "mouse trap". One or more controls located on the perimiter of the menu are easily accessed by moving (ballistically) far enough from the center of the menu, or by "pawing" the pointing device. This can be thought of as using the menu to reveal a gesture set to the user, where the gestures (e.g. long straight strokes) are made just prior to a press down event, which itself can be used to pop up more controls or initiate a drag. See George Fitzmaurice et al. (2003), Tracking Menus, Proceedings of ACM UIST 2003 Symposium on User Interface Software and Technology. Interestingly, there's also related previous work by Jens Tinz, called Radial Context menus, which I think of as a hybrid of tracking and marking menus.
• Another cool interaction technique: Hover Widgets, described by Grossman et al. at CHI 2006.

The above examples are only a small sample of the excellent and exciting research out there. They are not meant to particularly endorse certain works over others. I only list them here as examples of personal influences, in the hopes that they may inspire interest and possibly attract new people to this field.