I mostly agree with NoiseJockey.
I've been using found sound in my works since I started. The fact that I could turn the entire world into my instrument really excited me. I'll use doors shutting/slamming for kick drums, I'll layer slight hints of glass breaking to add some crunch to a snare hit, 2x4's being smacked together, metal pipe impacts drowned in reverb for drones or ambient textures, reverse it to create some nice tension building transitions... pots, pans, utensils, cutlery, kitchen droors opening/closing, TV's being smashed, random radio chatter, NASA transmissions, automobiles, random found electronic devices, mechanical parts or junk, dying hard drives, etc. It's all fair game to me. I'll use anything and everything. I've even thrown wet contact mics in pans of hot oil to see what I could get.
I shit you not, the deepest bass kick/LFE impact I've ever created was from smacking a metal trash can lid with a drum stick and then moving the lower lip of it to about 2 inches away from the condenser and just the vibrations coming off of it push so much air it would make Michael Bay's sound design team soil their pants.
If you don't know how, learn how to solder and how to make contact mics and hydrophones. They're really cheap/easy to make and absolutely invaluable for stuff like this, and also if you're a sound designer.
I'm even known to do crazy stuff like this.
This next one is really amazing though. I would have loved to be on location for this one with some shotguns and contact mics.
You should also check out "Symphony for Dot Matrix Printers" by [The User]. It's really impressive.
Actually, last summer I started a project that I'm (very slowly) working on with a composer out of New York named Jim Coleman. We made location recordings of places that are sentimental to both of us last summer while I was in NY. We spent about 3 hours making recordings on the Williamsburg Bridge where we were recording traffic, hammer, stick and pipe strikes or scrapes on the bridge, rails and sidewalk. We got the train passing by, random trash laying around, wind, etc that I'll be chopping up, processing, turning into Battery Kits and Kontact instruments to make compositions that are inspired by the feelings we get in those locations, made only from sounds of those locations.