Here is a cool way of knowing that there is someone at the door even before they knock or ring the bell! It is a nice early warning system for anyone who is being security conscious and that is just about anyone these days! It can also work for business users too, especially for people who are running their own business with no staff etc.
These are the features and specs for the excellent Doormat Chine;
Hidden, under doormat, security chime converts a welcome mat into a pressure sensitive pad that sets off one of three pleasant pre-programmed chimes for monitoring activity at your door. Portable bell unit works up to 100 feet away, and can be placed anywhere. Features volume control, and optional cordless doorbell unit. Requires three AAA batteries (not included) and one A23 battery (included). Vinyl, 19 1/4 x 12 1/2 x 1/2″.
Priced at around £25 in the UK and $40 in the USA
Source [Coolest Gadgets]