FUNimation also offers streams of some episodes of their series - in mind that the series on Crunchyroll's and FUNi's websites can range from G- to R-rated.
EDIT: There's a couple more I forgot:- a free online manga magazine Viz publishes.
It has an interesting mix aimed at older audiences.- another free online manga magazine from Viz.
It has preview chapters from some series, and fuller runs of others.-If you're a Rumiko Takahashi fan, a good place to visit.
It has chapters from Takahashi's latest manga series, Rin-Ne, available for free, and you can also catch episodes of the Inu-Yasha anime (both old and new series).is my favourite place to buy manga online.
I know that many people have mentioned websites in various group posts, but I though that it would be nice to have them all at one spot.