I love arduino.
That being said I misplaced mine somewhere in a physics lab and haven't quite had time to look around for it. I went ahead and ordered a TI Launchpad to experiment with instead. They could become my new favorite development platform since they're less than $5 each.
Adafruit is awesome and I buy stuff from them all the time. SparkFun is also great, and sells some arduino experimenting kits:
Starter Kit - $60Inventor's Kit - $95If you do any work at all with arduinos you will become very familiar with both SparkFun and Adafruit.
Arduino also has their own official kits:
Workshop Kit - €50Starter Kit - €80Kits are a great place to start but they can be pretty expensive. It's a lot cheaper if you buy the parts yourself but picking out the right parts, etc, requires some knowledge that you probably won't have unless you play around with one of the kits for a bit and start making stuff.
Dealextreme sells some Arduino compatible clones for
about half price. I haven't purchased any from them so I don't know how good they are. I assume they're probably fine, they just say "designed in Italy" instead of "made in Italy."
A motion censor would work if it were aimed at the door, but it would also pick up stuff like a paper falling off a desk or an insect. If you can set the sensor to pick up larger vibrations, you might be able to set it up so that it only picks up the ≥walking vibrations.
A
piezoelectric sensor is one way of doing this, and probably one of the cheaper ways. Here is one for
$1.50. A lot of the arduino kits come with them but they're buzzer style ones - it turns out if you pulse this with certain frequencies they play tones so a lot of basic projects involve playing "music" with your arduino. I've got a recording of mine playing ode to joy somewhere. In any case, they're not convenient for sticking on the floor as a sensor.
That flat one can also play music as well if you get bored of using as a vibration sensor, and if you build a cone for it out of paper it can be really loud, much louder than the buzzers.