Labyrinth Lord

From the back of the core rule book.

"Enter a world filled with labyrinths, magic, and monsters! You can take the role of a cleric, dwarf, elf, fighter, halfling, magic-user, or thief on your quest for glory, treasure, and adventure! This is a complete role playing game. All you need are a few sheets of paper and some dice. Welcome back to a simpler old-school gaming experience. The Labyrinth Lord awaits your arrival. Can you survive the dangers of the labyrinth?"

Labyrinth Lord is a retro clone game based on the Moldvay-Cook version of Basic and Expert Dungeons and Dragons.

This game is a "Retro-Clone". This means that it is a complete duplication of an existing, out of print rule-set, put out by a third party (although there may be some changes for legal or other reasons).

Retro-clones are legal because rules as such are not subject to copyright.

Other RPGs that fall into this category are:

Swords and Wizardry (based on the original 'white box' D&D)
Old School Reference and Index Compilation (OSRIC) (based on 1st edition Advanced D&D)
GORE (based on Basic Roleplaying; the system used by RuneQuest and Call of Cthulhu among others)
Zefrs (based on TSR's Conan RPG)