Labyrinth Lord is a role-playing game published by Goblinoid Games in 2007. It was designed by Daniel Proctor (using WotC's Open Gaming License) with the intent of consolidating the rules and recapturing the spirit of "the world’s most popular fantasy role-playing game!". More specifically, it's a retroclone of the 1981 D&D Basic and Expert set. A revised edition was released in 2009, and it has also been translated into German and Italian.
Any adventure module designed to be played with classic B/X D&D could be run using Labyrinth Lord's rules with little or no tweaking, and the rulebook itself is neatly written and superbly edited and organized. There are a few differences between Labyrinth Lord and its red-boxed forebearer, however. Most notably is that Labyrinth Lord extends the rules so characters can advance to Level 20 (the old rules only went up to Level 14). Nonetheless, for neckbeards who still want the “original edition” gaming experience, it boasts having Original Edition Characters, so you can play the original options under the updated Labyrinth Lord rules set.
Advanced Labyrinth Lords
A supplement called Advanced Edition Companion exists. Its goal is to make the game more compatible with Advanced Dungeons & Dragons. Advanced Labyrinth Lords is simply a compilation book containing both the basic Labyrinth Lord rules and the Advanced Edition Companion.
This supplement is great if you like the simplicity of the Basic/Expert D&D ruleset, with greater character options going beyond the restrictive 'race-as-class' setup for demihumans, and not have to deal with the bloated AD&D rule system.