fairly simple DTMF decoder to control solenoid door unlocker and connected it to phone (in this case a cell phone). NOTE:CALLERID would save the phonecall as commented on the article
a small device which plugs into a $20 prepaid mobile phone to make a GPS tracker. The Tracker responds to text message commands, detects motion, and sends you its exact position, ready for Google Maps or your mapping software
If your own water is safe from heavy metals, but you still like the way water tastes when it's been through something like a Britta filter, there's an easy way to convert the filter to be reusable, saving a ton of waste and a bit of money.