The Ancient Academy, a free one-page dungeon PDF from Robertson Games, after seeing it mentioned in a post on Dark Corners of Role Playing.
What I like about The Ancient Academy is that it presents a nice challenge to low-level characters--combat, exploration, and role-playing--yet the dungeon is generic enough that you can drop it almost anywhere in almost any campaign setting. Here is the back story and location information: 4 Leagues from the village of Thorn, the ruins of a monastery sit atop a rocky hill. Inside the last intact building is a staircase to the dark dungeon below. Very short and sweet. Just add water and heat for 10 minutes.
It is also system neutral, so you can use it with any early D&D edition or retroclone. You could also use it for GURPS DF or Pathfinder without modification (although you would have to add some ruleset specific information).