Quest of the Ancients.. from Black Unicorn Press (no longer around)
Okay that is obscure and for a good reason.
Aliens: The Roleplaying Game
Okay that is obscure
Recon from Palladium
Not that obscure. It was in print until Palladium had issues.
Villains and Vigilantes, Twilight 2000, Mechwarrior (RPG for Battletech), Warhammer Fantasy (the RPG, not the fantasy battles), and Tunnels and Trolls were popular mainline games for decades. Tunnels and Trolls has a current new edition. Those are not obscure, just not well known to the D20 saturated gamers of today.