Arrow and The Flash - Crossover coming in Episode 8 plus Felicity coming to The Flash

Thanks to Justin for the heads up.

As for how much The Flash will cross over with flagship series Arrow, Kreisberg told TVGuide.com after the panel that it'll happen often on a small scale, but they are planning for a big event later in the season. "Episodes 8 of both shows, Arrow and Flash, are going to be a two-hour crossover event," Kreisberg says. "[Also], in Episode 4, Felicity [Emily Bett Rickards] is going to come to The Flash."

Source:

TV Guide