I saw Maro post about a trap on YouTube today, so I thought I'd post one here that I just invented. Since we are friendly at WonderHowTo, the TNT has been replaced with a note block.
Start with the "bait-block", place a redstone dust on top of it.
Cause we need something to build on...
This will power the TNT. The torch can be replaced with a lever if you are short on redstone dust.
Place a regular piston (not a sticky one) like this.
Now we will turn the piston into a single-use BUD switch. Put blocks like this, any solid block will do, I'm just using wool to make it easier to see.
Remove the red wool. The piston will get an update, see that it has power from the green wool and extend.
Place gravel (or sand) on the extended part of the piston.
And the "TNT" like this...
The piston still see the power from the green wool so it won't retract.
Remove the green wool block...
...before you remove the rest (this is kind of important).
If the gold block is destroyed, the redstone dust on top of it will also be destroyed and, in turn, cause a block update to the block below the piston. The piston will react to the update, see that is no longer powered and retract. The gravel will fall down, let power go to the restone dust on the other side...by the TNT... Fssssss.... BOOM. :D
Of course, you can have more than a single TNT block, but it takes some practice before you know where you can place them. :)
"Yay! A free floating gold block! What harm could it possibly do to grab it?"