From The Flash to The Flash.
Fans familiar with the short-lived '90s TV version of the Flash immediately recognized the actor playing Barry Allen's jailed father in this version. John Wesley Shipp eventually took on the role of Jay Garrick, The Flash of the show's Earth Three. Shipp's fellow '90s Flash veterans Mark Hamill and Amanda Pays have also joined the Arrowverse, as their old characters The Trickster and Tina McGee, respectively.
Happy Birthday, Barry Allen!
Grant Gustin and John Wesley Shipp share more in common than their roles as the Scarlet Speedster. Both men were born in Norfolk, Virginia and both of them have January birthdays. Gustin was born in 1990 the year Shipp filmed his Flash pilot.
Throughout the fictitious town's history in the comics, Central City has been situated in Pennsylvania, Ohio, Florida, and Missouri. The Central City of the Arrowverse appears to be in Oregon. The show has used footage shot in Portland and in its second episode we see a map of Central City that's clearly based on Portland. (No word on whether or not any of the characters from Portlandia exist on that Earth...)
The Flash made room for Green Lantern.
Like its sister series, Arrow, The Flash has given us plenty of indications that Hal Jordan exists in the Arrowverse. We've seen Coast City, home to the Silver Age Green Lantern. There are numerous references in The Flash to Ferris Air, where Hal Jordan works. In the Season One episode "Rogue Air," Barry makes a remark about a test pilot who mysteriously disappeared; clear foreshadowing of Green Lantern's origin.
Harrison Wells is not from the comics.
The Flash and much of his supporting cast, locations, and situations are drawn from the rich mythology of the comic books, ever since the first version of the character appeared in 1940's Flash Comics #1. But despite the prominent role of Harrison Wells (and all of his multiverse variations) in the TV series, this is one character that was created specifically for the show. He's not alone. Eddie Thrawn, Tess Morgan, and Arrow's John Diggle were all created for the TV Arrowverse as well.
Tom Cavanagh isn't the only one to pull ArrowVerse double duty.
Plenty of actors on The Flash have played multiple characters, including multiverse doppelgangers and villainous lookalikes. But the man who plays Captain Singh on The Flash has an Arrowverse history that's less obvious than his castmates. Patrick Sabongui first appeared as a low-level drug dealer in the Arrow Season One episode "Vertigo" and again, as a random soldier, in Episode 17, "The Huntress Returns."
The Prison Break connection.
Wentworth Miller's portrayal of Leonard Snart was an immediate breakout (pun intended) on The Flash and it was a stroke of brilliance on the part of the show's producers to bring in Miller's old Prison Break costar, Dominic Purcell, to play Snart's partner-in-crime Mick Rory, aka Heatwave. This onscreen reunion actually led to the revival of their old show. As Miller said in a 2017 interview, "We hadn't seen each other in about five years. Suddenly we were back on set, discussing old times. And out of that conversation came the possibility of revisiting Prison Break."
The Big Bang Theory connection.
Sheldon Cooper is a well-known fan of The Flash and Cisco Ramon appears to be a fan of The Big Bang Theory. There have been several nods to the massive CBS comedy in the form of Cisco's t-shirts, which have included, "That's my spot," "Rock/Paper/Scissors/Lizard/Spock" and most notably, "Bazinga." (No word on whether or not any of the multiverse versions of Vibe are fans of Young Sheldon.)
The Candyman connection.
Zoom, the Big Bad of The Flash Season Two, was played by actor Teddy Sears but voiced by Tony Todd, veteran character actor from The Crow and Star Trek: The Next Generation, best known as The Candyman. Todd portrayed the supernatural killer in the 1992 horror film and its two sequels, Farewell to the Flesh and Day of the Dead.
The Glee Connection.
Of course a musical episode was inevitable for the Arrowverse! Both Grant Gustin and Melissa Benoist were part of the Fox musical comedy Glee. The Flash/Supergirl crossover, which gave us the best musical episode since Buffy The Vampire Slayer, reunited both actors with Darren Criss, who joined the Arrowverse as Music Meister. Who knows? Maybe Glee takes place on one of The Flash's multiple Earths.