Crunchyroll is one site... But it's not perfect since a lot of shows (if they have a lot of episodes or are older) might be hidden behind a premium account type (which requires a payment).
Youtube also has some shows... But not all of them might be visible in certain regions.
It'd be interesting to see other sites that stream anime also.
you got to be careful though on youtube, most don't have full series or the video cuts out half way to a ad about joining up on some site to see the rest.
Sometime ago I used to go to Anime Freak, they have a decent amount of anime and won't have you opening too many windows or downloading different players. You will have a few advertisements but it is a free site so I think it is a fair deal.
Complementing on crunchy roll, i believe there is a free mobile application so if you use your cellphone often that might also be a choice.