Justice League, on Cartoon Network
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This ran for two seasons, from 2001-04. It's about the famous group of seven superheroes working as a team: Batman, Superman, Flash, Green Lantern, Wonder Woman, Martian Manhunter, and Hawkgirl. I suppose they still do a lot of stuff by themselves, but of course that's not what this show is about. It's about them working together. It's part of the DCAU, which began with Batman: TAS and then Superman.
Justice League Unlimited
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In the third season, the show changed its name to "Justice League Unlimited," as many new members were added to the League. This would run for three more seasons, from 2004-06. (I consider JLU to be the same series as "Justice League," although for the most part I think this part of the series is better than the earlier part, and my rating of the series overall is largely due to the JLU seasons.) An interesting new story arc was introduced, about Project Cadmus, and the government covertly working against superheroes. And of course there would be other stories in JLU, as well. I should probably say more about the series, I just can't think what. (No doubt I'll expand my review whenever I get around to buying the series on DVD and rewatching it.)