Do you have an older webcam laying around the house or garage that is no longer being used? Well, dust it off and put it to a renewed use. Monitoring software is available for free or as trials and some for a pretty small price ($19) so it will come in handy. There are options to use them with PC, Mac OS X, or even Linux.
PC:
http://www.tincam.com/ (Free Trial/$19)
Mac OS X:
http://evological.com/evocam.html (15-Day Trial/$30)
Linux:
http://www.zoneminder.com/ (Free)