I cannot believe we've gotten 8 pages into this thread and nobody's mentioned Teen Titans yet.
It's Robin (minus Batman) as leader, with Cyborg, Beast Boy, Raven, and Starfire as a super-team who lives in a giant T-shaped headquarters and saves their city from villains and stuff. It's got a funky limited neo-anime animation style that takes some getting used to, but I was a huge fan of it back in the early 00's when it came out. For a cartoon aimed at kids, it tackled some pretty heavy themes- betrayal, death*, and destiny- that sort of thing. There was an ongoing arc in the final season that had to do with Raven being sent to Earth to serve as a portal for her demon father (and source of her power) to take over the planet that was pretty relentlessly dark.
Oh, and also Tara Strong was the voice of Raven. Raven had a really low voice, though, because she was angsty and half-demon. Imagine Twilight Sparkle has a really bad sore throat and is angry and sarcastic about it, and that's Raven. (There's actually several scenes where Raven's using magic to levitate books around herself in her room, and she's the most anti-social one of the team. There's definitely some parallels there.)
They even got Wil Wheaton to come in and voice Aqualad for a few episodes.
You can watch Teen Titans on Boomerang late at night, it's worth throwing a couple episodes on the DVR to check out if you've never seen it before. Do yourself a favor and don't go searching Youtube for it, though- WB pulls down full episodes, so all that's left is mostly fan-created music videos set to slideshows of even-worse fanart. The only reason to watch -those- is to realize how good we have it with ponies by comparison.
Oh, hey, speaking of Teen Titans fanart: Have a seven-year-old piece by our own Artrix, which came up entirely coincidentally while I was GIS'ing "teen titans" so you don't have to:
*comic book death- the dead character comes back later on in the series. It's played pretty straight at the time, though.