Batman / Bruce Wayne (Wayne and Queen merge companies according to Reverse Flash's future newspaper.)
Slingshot / David Drayson (on Robert Queen's list.)
Electrocutioner / Lester Buchinsky (on Robert Queen's list).
Jericho / Joe Wilson (Deathstroke mentioned his son Joe in season 1 of Arrow.)
Dr. Arthur Light (When Deathstroke attacked STAR Labs in season 2 of Arrow, Cisco slows him down using a gun designed by Dr. Light. Caitlin mentions that he was an ex-employee of STAR Labs but was fired by Wells for being a psychopath.)
Elongated Man / Ralph Dibny (mentioned as one of the people who died/were injured during the night of the particle accelerator explosion.)
Amazing-Man / Will Everett (mentioned as one of the people who died/were injured during the night of the particle accelerator explosion.)
Fire / Beatriz de Costa (mentioned as one of the people who died/were injured during the night of the particle accelerator explosion.)
Damage / Grant Emerson (mentioned as one of the people who died/were injured during the night of the particle accelerator explosion.)
Blue Devil / Daniel Cassidy (There were ads for a Blue Devil movie in Arrow. Iris and Barry actually watch the movie in The Flash.)
Guardian / Mal Duncan (Barry mentions that Mal Duncan is playing at a jazz club.)
Aquaman / Arthur Curry (Jay Garrick mentions having a friend from Atlantis. Was actually referenced in a deleted scene from the very first episode of The Flash.)
Nightwing / Dick Grayson (They've referenced Bludhaven a few times.)
O.M.A.C. (Ray looks at files for top secret weapons called OMAC. OMAC is never built though, since Ray used the designs to build his Atom suit instead.)
Booster Gold (Rip Hunter's dad.)
Star Sapphire / Carol Ferris (We've seen billboards for and planes owned by Ferris Air.)
100% not canon since the lines never made it, but an early draft of of the pilot of The Flash references Stargirl and Solovar.