• Carbon monoxide detectors
• If you live in a radon-prone area, appropriate detectors
• Motion sensor flood lights strategically mounted on exterior
• +1 on Jeff's fire extinguisher recommendation – often overlooked.
• Decent dead bolt locks on exterior doors.
• Sliding glass doors and windows equipped with "anti-jimmy" devices.
• If it's overkill (or too expensive) to have an alarm system that's monitored by a service (ADT, Brinks etc), there are plenty of packaged systems (without an outside service) on the market.
• For the really thrifty, buy
these instead — window decals and yard sign — that make a bad guy
think you've got a monitored alarm system.
• The firearm of your choice close at hand.
• Braided stainless steel burst-resistant water hoses for clothes washer and refrigerator ice-maker (I installed after my bro-in-law had an "incident" while away on vacation). Believe may be mandatory in some municipalities.
• Earthquake straps on your hot water heater, even if you don't live in traditional earthquake country. <$20 for a kit.
• Is your garage well-secured, especially at night? (all too often an easy access point that's overlooked).
• "Mold" is a hot topic these days. If you have a crawl space that's moisture-prone, or other mold-friendly conditions, might be worth a check-up.
• If you live in an older home (aging electrical and plumbing; construction pre-dates important building codes), and you don't have the knowledge to do it yourself, consider paying $250 +/- for a *qualifed* home inspector to give it the once-over...a small price if it reveals significant shortcomings. Or, Google "home safety inspection checklist" and check out the 1.8 million hits.