this is a parachute release mechanism for a model rocket, i built it for a water rocket but other types would work. It uses an Arduino pro mini (http://www.sparkfun.com/products/9218) which is really good for this as it weighs less than two grams and is tiny. The mechanism fits in a 2 litre drinks bottle which can be attached to the top of your rocket, you could easily scale this project up or down for different sized rockets.
the device works with a tilt sensor, when the rockets power tails of then it will tip forward as the front is heavier, this will (hopefully) trigger the tilt sensor which will turn the servo releasing the door .
the frame that holds the parachute is made out of cardboard, i drew around the bottom of the bottle and cut it ouut then used it as a template for the other side. i also cut a long piece of card slightly thinner than the diameter of the circle and cut a rectangular hole in the middle of it for the servo. Push the servo through this hole and screw the screws into the card. I used superglue to attach the two cardboard circles to the strip (like in the picture). i cut out four cardboard triangles to support the circles and glued them in place. i made a flap that goes over the servo to protect it from the parachute and made two boxes of card for it to rest on and glued a bath sponge cut in half to the flap, this is so that when the door on the bottle opens then the parachute is pushed out.