Crunchyroll has a lot of the latest anime series coming out of Japan like the latest Shonen series as well as classics like Mobile Suit Gundam and Fist of the North Star.
Hulu has a ton of series too, just search "anime" and you can go to the genre page and check out what they've got. If you subscribe to Netflix there's lots of stuff on there, but it isn't free. Lastly, there's YouTube, which has a surprising amount of material as will. Just search around there is a lot of stuff legally streaming, I keep a bookmark folder to track a bunch of anime that haven't seen that are streaming as well as old favorites.