Taken from: Recycle Now, an English website
Use in your garden as mulch, or between rows of plants to prevent weeds - can be turned upside down to show a nice webbing effect. Once weeds start to pop through, you can lift up the carpet, breaking the weeds and then let it down again!
Cut up carpets for use as foot mats in cars or to line the boot (trunk in the US!)
Cut into small circles and place under the feet of heavy furniture. This should help to protect your new carpet!
Take old mats or rugs on camping or caravan holidays to use as a doormat, either outside or inside the caravan (camper).
Old pieces of carpet can be used to cover your compost bin over the winter
If the carpet is in excellent condition, contact Habitat for Humanity or the Salvation Army to donate.