Evological's EvoCam program is perfect for whatever you need. I've used it with my iSight for survallence and it works just great. You can have it record video only if it detects motion, take a series of stills if it detects motions, just take pictures every whatever minutes, and then upload the results to servers, save them in whatever folders, or use the computer itself as a live webcam server that's continuous.
All that and more. You can customize everything about the way it works. There's a demo for 30 days, but it's well worth the money.
EDIT: My favorite part is that you can set up motion sensors for just certain parts of the picture, so, if you put a sensor area over your doorstep, it'll take a picture when someone walks there, but if you have a plant that's moving with the wind outside of the sensor, it won't take a picture. You can also set sensitivity.