I designed nearly the best tnt/falling sand trap. Use at your own risk. Not responsible for death, griefs, bans, etc.
Materials:
1 TnT block
24 Signs
In a biome where sand is bountiful (obviously the desert)
1 Stone pressure plate
Shovel/pickaxe
A bit of time
Start by digging a 5x5 hole in the sand. Make the hole 5 blocks deep also. in the center, 1 block under the surface, place a tnt block. surround it with signs. place sign over sign, until all blocks above can be supported with a sign block. Note: since the signs should be attached to the TNT, when the tnt is set off (primed), the signs will fall, causing the sand to fall. this sends the user falling into a deep pit (make it deep as you want), and they have no chance of survival , because the tnt will fall alongside the player and detonate. The pressure plate will be stone, so there's no "feathering" with blocks. If you make the pit big enough, then it is possible, but why waste that effort? This trap is guaranteed to kill if done correctly, and should drop the players loot, if they survive the initial impact the ground. Since it only uses 1 tnt block, it is extremely resource friendly. Better yet, if you can find a material that will do the same, you can make a pitfall trap.
Use at your own expense. This guide does not protect you from the evil grasps of moderators. Do not attempt if you're not looking forward to a nice ban.