One of the classic home automation projects is knowing when the mail carrier has left mail in your delivery box. (See Smart Home Hacks for many others.)
A fresh new approach to solving this problem (which usually involves wireless motion detectors) is discussed at Make's Snail mail push alerts.
The hack uses an Arduino board and the fantastic Prowl notification service for iPhone. As noted in the discussion, the technique is easily adaptable to other problems, and given the relatively low cost of the components used, you can bet that I'll be deploying more than one.