My greatest trap idea which I've never actually gotten around to building go thus:
Main floor:
.+%%.
.+%%.
.+%%.
.+%%.
.+%%.
.-channel
+-floor
%-screw pump(pumps from east)
This involves elements of the water cannon. The water is shot via screw pumps(not sure if this would work, perhaps you need a pressure tower?) across the 1-wide hallway, and the goblins fall into the trench on the left.
One level below:
█#█≈
█#█≈
D#█≈
█#█≈
█#█≈
█-wall
#-grate
≈-water
D-locked door
The water on this level is meant to feed the screw pumps. What's important to note here are the grates to catch the goblins. This lets all the water out, while still keeping the goblins intact. They're not dead yet, but they soon will be. The door is so you can access all their stuff after...well, read on.
The level above
█H█
█H█
█H█
█H█
█H█
█-wall
H-Hatch cover
This level is crucial to the entire design, as those hatch covers are filled to the brim with fiery hot magma. Obviously, they and the mechanisms that operate them need to magma-proof, otherwise this will be a one-time shot. You'll also need some way to refill this tunnel of magma for re-use.
Now, what happens is as follows: the water cannon propels the goblins onto the grates. The water drips down and muddies the ground of an evaporation chamber at least 2 z-levels below everything (diagram not included). Then, the hatches are opened and the magma falls onto the heads of the goblins. The goblins are burnt to a crisp along with all their useless cloth as well. Their iron is left behind for you to access at your leisure (wait till the fire's gone out, though). The magma also passes through the grates, falling on the water below, creating obsidian. If you have discovered a cave river/pool, your evaporation chamber will also become a tower-cap farm. Otherwise, it can be used for regular farming.
Therefore, you:
A) Kill goblins
B) Get their iron stuff
C) Don't have to worry about their useless clothes
D) Get obsidian
E) Get tower-caps (cave river required)
Your mileage may vary. As I said, I've never actually tried this out.