Pictured to the right is the world's cheapest fish trap. Click on the image to for a higher resolution shot. I'm sure this has been done before.
It is made from a plastic soda bottle, a couple of push pins, and a bit of fishing line.
(1) Cut off the top of the bottle where the cap screws on.
(2) Cut the bottle in two a couple of inches down from the top of the bottle. You want to make this cut just after the main cylinder of the bottle's body starts. The one pictured is actually cut too far down the bottle by about an inch. Oops.
(3) Invert the top of the bottle and shove it into the bottom of the bottle. You don't need to shove it all the way in. As pictured, it just needs to go
(4) Use the push pins to secure the top in the bottom. Wire would work, as well.
(5) Cut some holes in the bottom of the bottle.
(6) Thread the fishing line through the holes and tie.
(7) Throw a few bits of dog food or fish food into the bottle.
(8) Submerge in a lake / stream where there are small fish.
(9) Tie the fishing line to something so your trap doesn't wash away.
We had fish in our trap within a few minutes. Since it is pretty small, you'll want to check and empty often. It works because the fish can't seem to find the hole to swim out, though they will randomly find it every now and then.
If the bait floats, I have found that orienting the trap such that the opening is aimed a bit down works better in that the food floats to the back of the bottle, giving the fish clear line of site from the opening to the food.
If you want to catch larger fish, use a bigger bottle. If you want to catch really large fish, it wouldn't be hard to build the same kind of a trap out of chicken wire or wire screen. Certainly, there are likely laws governing the use of traps above a certain size.
I ran across Dead Pr0grammer's Cafe this evening.
As a former New York City resident and a programmer, Dead Programmer's Cafe is some powerful good stuff. Interesting writing and awesome photography.