I had a lot of hard metal duct (as in not the flexible stuff) left over from re-routing the ductwork in my basement to the sides rather than right down the middle to make more headroom. I had been wanting to upgrade from the cheap tin-foil crap that the previous owner of our house had used to vent the dryer to the outside. This job was particularly bad (splice in the middle, excess length, crappy cap on the outside, etc...) but it was just another one of those things that after three years in the house I still hadn't gotten to. Yesterday I took two 4" elbows and a long section of 4" pipe that used to deliver air to the bathroom from the main trunk line and combined them with a dryer vent I bought at Lowes. The old vent pipe was a fire waiting to happen, plus the buildup had slowed the airflow down to a crawl. Now the airflow is significantly stronger and clothes dry a lot faster. Granted I had most of the materials laying around so it hardly cost me anything, but to buy the materials brand new would still be very cheap. The energy savings are great as well as the safety factor.