[Correction: updated description]
Here’s another neat video that Tim found. It features a joystick that you can use to point its “missile shooter” towards a target. Here’s the description:
NXTShot 2.0 is lego NXT model cannon. Its targeting is driven in realtime by a Mindsensors Acceleration Sensor mounted on a remote. Its firing mechanism is powered by springs from old BIC pens. It has an innovative homing system that sets the limits for turning and inclining using a single light sensor. It has an onscreen menu system that uses a voice to give feedback on selections. It is programmed in ROBOTCC.
I have made plans of NXTShot using Google SketchUp and the fantastic NXT components provided by Payton White (see tinyurl.com/yw7tnw). More pictures of NxtShot can be found at tinyurl.com/2l7m2m.