This game (Secret Hitler) ruined quite of few nights when I was an exchange student: people kept playing it while it was limited to only 10 players IIRC and was an obvious albeit less-fun rip off of the game "Les Loups-garous de Thiercelieux"[1].
If Secret Hitler is a ripoff of anything, it's Avalon, which is similar to Mafia/Werewolf, but is much deeper and lacks the issue of player elimination.
I'd say it draws more from The Resistance/Avalon Resistance than Werewolf, since it focuses on voting and scoring points more than killing players and eliminating them from the game.
As a former member of the non-US audience who don't know what exactly is this Cards Against Humanity, I can confirm it's better to remain under the bliss of ignorance.
Aside from what everyone else has said, it's also a shameless knock-off of Apples to Apples, which preceded it. It's basically the same game, but more profane.
I'd call it an "intelligent continuation" rather than shameless knock-off. Apples to Apples targets innuendo for a family setting, CAH is a very adult game.