I thought I'd mention a couple of the entries to the One-Page Dungeon Contest that I've read. Check these out!
Mike Monaco did The Belly of the Beast, a living dungeon. Dyson Logos did a similar dungeon, Into the Worm's Gullet, although this dungeon is not currently living. Both are pretty challenging, although Mike's dungeon is probably moreso, what with the whole demons of the Abyss thing going on.
Roger Giner-Sorolla did The Egg of the Gazolba, which might work for beginning characters, despite the unbeatability of a couple opponents, because you are supposed to solve the problems with cleverness rather than combat. The whole set-up of kings who wear living birds who lay eggs on their heads made me think of Doctor Seuss, for some reason.
Tim Shorts did Race to Adventurer's Keep, which interestingly enough does not use a map. It feels sort of like a random Choose Your Own Adventure, with characters passing through three or more encounters before reaching the goal. I'm pretty sure Tim intended this as a beginner adventure, since the set up is that the race is the "final exam" for characters training as adventurers.