Supergirl just made her first appearance in the Arrowverse! As Barry Allen, Cisco, and Dr. Harrison Wells travelled to Earth-2 in tonight's episode of The Flash, we caught a glimpse of Green Arrow and The Flash, and finally the Girl of Steel herself.
There's been some speculation that that's Justin Hartley's Green Arrow from Smallville, but I'm not convinced that's the case. If anything, it looked more like future versions of the two heroes in slightly different costumes, but we'll all need to rewatch the episode over and over to be sure! For now, we at least have the gif below.
Supergirl showing up here (even if it is just footage taken from the pilot episode of her CBS series) is awesome, and a precursor for next month's crossover. It looks like there are still plenty of alternate Earths for The Fastest Man Alive to explore anyway...