Your Most Memorable D&D Trap!

[Raith Shadowsoul]

Yeah, I've never actually done that to any of my PC's before (Sirith being one of them.) However, I will now, mwha.

[Sepoko]

There's a cylindrical room with seven floors. The top floor is only a small platform with the door that leads into the room. The next floor down is a ring of spikes 120' below the platform, going all along the wall of the cylindrical room. The next is another 120' drop into spikes, and so is the next... and the next... and the next. This goes all the way to the bottom, where more spikes await the players, along with a secret door to the next area. By the way, the room is filled with multiple beholders only too happy to use their anti-magic fields to lend flying players "a helping hand" down a few hundred feet into spikes.

Oh, I almost forgot to mention this last part. I poisoned the spikes.

[The_White]

Basic one. Large, obvious, wide pit. Invisible bridge.
If you use it a few times there are some good modifications to the basic theme.

1: Bridge has nothing guarding the edge to stop users falling off
2: Bridge changes direction part of the way across (starts going diagonally or something)
3: There is a small (jumpable) gap at the start of the bridge
4: There is a gap in the middle of the bridge
5: any combo of the above
6: any of the above + the PC's are being chased by something HUGE