The Clockwork Nirvana of Mechanus is the Plane of Lawful Neutral in Dungeons & Dragons and its Great Wheel cosmology, introduced to the multiverse in the Advanced Dungeons & Dragons setting Planescape.

Representing the pure perfection of order; order over everything else. As the pinnacle of structured thought and action, Mechanus appears as an infinite array of clockwork machinery. In this vast plane-machine, life clings to gears the size of continents and pistons the size of planets, completely in tune with the ticking of the multiversal clock.

Mechanus's most iconic inhabitants are the Modrons, geometrically shaped extraplanar beings fashioned of otherworldly machinery. Come third edition the modrons lost their place of prominence and were supplanted by the Formians and the Inevitables, something explained officially as being due to the death of Primus at the hands of Tenebrous (the undead Orcus), as detailed in the adventure "The Great Modron March". Unofficially it's held that modrons were pushed out of the limelight because they were seen as embarrassingly goofy by Wizards of the Coast. Fortunately, like other goofy old-school things, they were rescued by 5e, restoring them to their full, somewhat silly, glory.

The Mechanatrix race is the Mechanus-descended equivalent of Aasimars and Tieflings, being genetically something like 99% human and 1% steampunk robot.

Pathfinder's analogue to Mechanus is Axis.

The Outer Planes of the Multiverse
Seven Mounting Heavens of Celestia Twin Paradises of Bytopia Blessed Fields of Elysium Wilderness of the Beastlands Olympian Glades of Arborea
Peaceable Kingdoms of Arcadia The Upper Planes Heroic Domains of Ysgard
Clockwork Nirvana of Mechanus The Middle Planes Concordant Domain of the Outlands The Middle Planes Ever-changing Chaos of Limbo
Infernal Battlefield of Acheron The Lower Planes Windswept Depths of Pandemonium
Nine Hells of Baator Bleak Eternity of Gehenna Gray Waste of Hades Tarterian Depths of Carceri Infinite Layers of the Abyss