One of the technologies that home automators are watching with interest is "ZigBee." (It's an advanced wireless technology.) Unfortunately, software support and affordable modules have been slow in coming; it seems that the ZigBee folks are pursuing the more lucrative commercial automation market instead of the home do-it-yourselfer. (A decision I can't really fault them for, if it's true.)
But that might be starting to change. The recently introduced Home Heartbeat Starter Pack is ZigBee-based, reasonably priced (comparatively speaking), and offers a wide variety of options for monitoring your home. You can watch for drawers being opened, water leaks, the garage door opening, and the on/off status of key appliances. This information is available to a nifty-looking keychain remote, or you can have it sent to your cell phone over SMS when you're away.
It appears, however, that the Home Heartbeat Starter Pack only includes one module, not all of those pictured in the product description. So, the price will grow if you want a more sophisticated system, but this also allows you select only the pieces that you need.