Ralph refers to The Monitor as their "Asgardian cosplayer", referencing Marvel's Thor who is an Asgardian from Asgard.
Ramsey Rosso tells Barry that he got his dark matter from his "old friend" Ted Kord, who in the comics is also known as the Blue Beetle.
Ralph's mom Debbie has a colorful list of ex-boyfriends, but one name jumps out specifically: "Marv Perez". a clever riff off the names Marv Wolfman and George Perez -- the writer and artist for the Crisis on Infinite Earths comic book limited series.
Mitch's Romero's surname is a reference to George A. Romero, a filmmaker best known for creating the Night of the Living Dead franchise.
When Nash Wells confronts Cisco, he waves a sizeable ruby at him and tells him that the stone gives horrible nightmare hallucinations to all its victims. In the comics, Morpheus from The Sandman has a dream-manipulating ruby.