Motion

Motion uses a video4linux device and detects changes in the image. If a change is detected a snapshot will be taken.
Some of motion's options include:

Sending an email when detecting movement

Sending a SMS message when detecting movement

Execute external commands when detecting movement

Take automated snapshots on regular intervals

Take automated snapshots at iregular intervals using cron

Create mpeg movies of detected motion
More info on their homepage !archive.org