Netflix has just added a plethora of wonderful Cartoon Network shows! Most of these seem to be from early 2000s, but there is some newer stuff here too. Definitely something for everyone!
Justice League (2 Seasons)
This show ran from 2001 until 2004 and is considered one of the greatest super hero cartoons in existence. Personally, I've always preferred Marvel cartoons, but this show really does shine. It's no Batman the Animated Series, but it sure is good. Netflix has both seasons for a total of 52 episodes. I know it is coincidence, but DC sure does like the number 52!
Justice League on Netflix
Justice League Unlimited (2 Seasons)
To me, this always seemed like a bit of a forced sequel to Justice League. I don't mean that in a negative way though. Justice League Unlimited picked up in 2004 where Justice League left off. It features a huge cast of Justice Leaguers, and is really worth watching, but watch Justice League first. Netflix has them both, no need to watch them out of order! The last episode aired in 2006.
Batman Beyond (3 Seasons)
From Paul Dini, Batman Beyond is a kind of continuation of Batman the Animated Series. You don't necessarily have to have seen Batman the Animated Series to understand this, it's kind of it's own thing. The story follows Bruce Wayne as a senior citizen, still trying to fight crime, but beginning to realize he is getting too old. A kid named Terry proves himself and Bruce eventually begins training him as a new Batman. It's not as good as Batman, but it's certainly good.
Batman: The Brave and the Bold (2 Seasons)
This show is essentially DC Team Up. Each episode features Batman teaming up with somebody in the universe to stop somebody else. The show can be hit and miss, but overall I would give in a whopping "okay". Maybe Batman the Animated Series has raised my expectations so high I'll never enjoy another Batman cartoon.