The World's Smallest Dungeon is "an adventure for one first-level character." It is a scenario that is SRD compatible and OGL compliant.
From the back of the booklet:
This fully playable adventure, suitable for a first level character of any race or class, is a perfect introduction for new players.
Containing two encounters and a full sized map suitable for use with miniatures, it can be played in less than a half hour.
So, cough up a dollar, grab some dice, and get read to enter...
The booklet does have a listed MSRP of $1.00 and is only 6 pages long.