Rebels airs on DisneyXD. If you subscribe to the channel from certain TV providers, you can stream all of season 3, free, right from the source:
http://watchdisneyxd.go.com/star-wars-rebels
(Season 3 is still airing now, episode 16 was this past weekend)
Unfortunately, if you want to watch the earlier seasons, I think you have to buy the DVDs because they're not available on Netflix or Hulu. You can also stream them from Amazon, iTunes or YouTube, but have to pay for it.
It's not available anywhere for free, other than recording the episodes as they air live. (via legal sources, anyway)
Some good info here, if you're just thinking about wading into Rebels.
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