Nightsisters of Dathomir
The Courtship of Princess Leia may not rank among fans' Expanded Universe favorites, but it did introduce an incredibly cool concept: the rancor-riding Witches of Dathomir, whose evil Nightsisters clan has survived into current canon through The Clone Wars (minus the rancor riding).
In Galaxies, the MMO from Sony Online Entertainment, the planet Dathomir hosted both the Nightsisters and their rivals, the Singing Mountain Clan. Neither were friendly.
For Empire at War: Forces of Corruption, players joined forces with the Nightsisters and eventually recruited one of them, a character created for the game named Silri. She wielded a lightwhip and commanded a rancor named Cuddles.
Meanwhile, the canonized Nightsisters allied with Count Dooku and their former member Asajj Ventress to play a prominent role in a (likely non-canon) Nintendo DS game called The Clone Wars: Jedi Alliance.