Can't believe I haven't seen this one yet:
Alysson Hannigan for Buffy and How I Met Your Mother.
Also:
Eliza Dushku- Buffy and Dollhouse, and Tru Calling
Continuing with the Arrowverse:
Grant Gustin - Glee + The Flash
Katie Cassidy - Gossip Girl + Supernatural + Arrow
Willa Holland - The O.C. + Arrow
Caity Lotz was on Mad Men, I believe, though I don't know how big her roll was. So was Daniel Panabaker, I think. They went on to be on Arrow and Legends for Lotz, and The Flash for Panabaker.
John Barrowman - Dr. Who + Torchwood + Arrow/Legends
Neal McDonough has been in a ton of stuff. Along with his roll on Arrow and Legends, he was also in Suits and Yellowstone most recently.
Katrina Law - Spartacus + Arrow
Manu Bennett - Spartacus + Arrow
Colton Haynes - Teen Wolf + Arrow
Tyler Hoechlin - Teen Wolf + Supergirl
Wentworth Miller and Dominic Purcell- Prison Break and The Flash/Legends
Callista Flockheart - Ally McBeal + Supergirl
He'll always be Thomas Andrews in Titanic to me