If you don't want to monitor your home by building your own system, and you live in Canada, then this news might interest you.
Bell Canada is introducing a wireless home monitoring system for about US$20 per month. It will alert you to events, such as your children coming home or localized flooding, via SMS or email. With additional equipment it even offers a live video stream so you can check on the situation remotely.
Thanks to the XTension list for the link.