USB web camera motion
There are 2 different technologies with similar names motion detection and motion tracking.
There several applications that will turn any webcam into motion sensor. Here is a list of motion detection software. Several years ago Logitech even included motion detection software with any sold webcamera.
If you want a super simple solution that does not require any new software – consider using online motion detection service. With any browser you can turn you web camera into motion detection home security system in 5 min.
The big limitation of using USB web cameras is the length of USB cable and the fact that your PC must be always turned ON. Using wireless IP cameras will solve both issues – but decent IP camera (like Axis M1011-W) start with $200. There are couple devices on the market that give you wireless USB – but their wireless connection is not as reliable as Wifi, so I would recommend avoiding wireless USB till it matures.
Motion tracking means that the camera will actually rotate to follow any detected motion in the room. Most useful for tracking your own face when you talk on Skype. Not any webcam can do this. Most popular webcams with motion tracking is Logitech Orbit and Microsoft Live Motion. Both USB web camera motion sensors and software are good enough for home users.