30 Day Challenge: Day 13 - Favorite trap or puzzle
While traps and puzzles are an old-timey D&D staple, I've never liked them. Either as a player or as a GM either one. I don't think that they're fun. They're usually frustrating, and certainly extremely gamist. They also don't really reflect much of anything in the fantasy literature from which the game sprung, or which some of it's players (i.e. me, at least) like the game to more closely resemble.
That said, there is one "trap" concept that works quite well for me. Haunts, from Pathfinder, first released as a 3.5 mechanic, but later ported into Pathfinder. It would work well in any d20 game, and is also easily enough adapted to any other game, for that matter.
Haunts really represent some elements of the more classic ghost story. They don't represent an actual ghost per se, but most ghost stories have all kinds of creepy events that work, if you really dig into its nuts and bolts, more like a supernatural trap than anything else, and haunts are this exactly.
Here's the link to the relevant section of the PRD, so you can read how they work.