The Woolies -- Another Season 1 callback, the Woolies are a sentient race who were subjugated and set to perform hard labor by the Chritchellites. Jack saved their butts in "Jack, the Woolies, and the Chritchellites."
Olivia and the Ravers -- She's the DJ at the rave Ashi discovers, and Jack saved her as a small child from a mind-controlling DJ in the episode "Jack and the Rave." Ashi's dance in this episode also echoed Jack's dance from the original episode.
Da Samurai -- He was an aspiring, and funkier, samurai that Jack met in his travels, when he completely embarrassed Da Samurai in "Samurai versus Samurai." Keegan-Michael Key voiced him in this episode.
Demongo, The Soul Collector -- He enters Da Samurai's bar and went toe-to-toe against Jack in his own Season 2 episode "Jack versus Demongo, The Soul Collector."