This is the simplified list for easy viewing order for Arrowverse!
DC's Legends of Tomorrow
Webisodes(you can find them at youtube):
Blood Rush - between Arrow S2E05 and S2E06.
Chronicles of Cisco - between Flash S2E17 and S2E18.
Superhero Fight Club 1.0 - is a promo for the final episodes of Arrow s3 and Flash s1 (arrives after Flash S1E18)
Superhero Fight Club 2.0 - is more or less a promo for Supergirl joining CW. (arrives before Flash season 3)
Constantine has cameo in Arrow S4E05. I do recommend you to watch this series, but there's no must. But he might get back into the Arrowverse one day. (and I also would recommend to watch it in this order: 1, 6, 2-5, 7-13. Episode 6 was shot to air as the second episode of the series)
I have added SupergirlSeason 1 before the crossover episode to the list. You can watch it basically whenever you want, as long as you watch it before Flash S2E18 or after Flash S2E18(I personally prefer to watch it before Flash episode, but both ways work). Also S1E04 and S1E05 were aired ouut of original order. Episode 5 should be viewed before episode 4
Freedom Fighters: The Ray - 2017 (This animated series will follow Raymond "Ray" Terrill from Freedom Fighters and it will air on CW Seed as Web series like Vixen).
There are also some comic books that are following the series:
Arrow - takes place turning Arrow season 1
Arrow - Vengeance - takes place turning Arrow season 2
Arrow: Season 2.5 takes place between Arrow season 2 and 3
The Flash: Season Zero - takes place turning Flash season 1
Arrow: The Dark Archer - takes place at the end of season 3 of Arrow
The Adventures of Supergirl - takes place turning Supergirl season 1