Not only does this trap actually work, it is humane! I know most of you probably don't care about killing a few dozen flies or so, but if it is at all possible to get by without killing something, I am all for it. Just ask the wasps that live on my porch... (I did give them their evacuation notice, but they haven't taken heed yet. I may have to take more drastic measures and relocate them myself once it gets colder.)
In any case, this trap is simple to make and works like a charm. Simply take a small jar and a piece of paper. Twist the paper into a fairly wide cone with a hole in the tip just big enough for a fruit fly to pass through. You may have to fiddle with it a little; when you place the cone in the jar, the tip should not be touching the bottom of the jar (or your bait). Tape the loose edge of the cone to secure it. Set the cone aside and place a bit of fruit or fruit peel in the jar. Fruit flies go crazy for cantaloupe and banana, and less so for orange and watermelon, in my experience. Gently set the cone in and secure it with two or three pieces of tape, making sure there are no gaps between the cone and the glass. Set the trap wherever you notice the most fruit flies. Obviously, it will help to remove any other attractants in the room. Do not let the trap sit longer than overnight, since the idea is to get rid of your fruit flies, not breed them (you also do not want to reuse your bait for the same reason). Once you have caught yourself some of those bitty buggies, take the jar outside and release them.
For a picture tutorial, go HERE.