I use quite a few sites for this~
First one would have to be
www.Otakucenter.comThat site usually has any anime I'd want to watch, and is updated regularly. The userbase is enormous, and has tons of uploaders.
The episodes are hosted on sites such as Myspace, Veoh, Imeem, Megavideo, as well as a few other ones I can't name off the top of my head.
This is my first choice when looking for an anime.
Second would have to be
http://www.ani-haven.net/2007/02/stream-section.htmlThis site links to Megavideo, from what I've seen.
Third on the list would be
http://www.zomganime.com/Rarely does it ever get past this step.
But, in case it ever does, I have some back-up sites:
http://www.anivide.com/index.html?list=anime <-- Pretty large list of anime. Links to many other sites~ A very nice site. <3 EDIT: Turns out dawood mentioned this one, as well. In that case, I vouch for the validity of the site.
http://www.animeseason.com <-- Another site I use as a back-up~ EDIT: Once again, vouching for validity~ Thanks ya Dawood~
http://www.anilinkz.com <-- I use this site to watch new Gintama episodes. Nothing else, nothing more. =X
Although, from what I gather it's a pretty good site as well. Offers mirrors of the episodes just in case one doesn't work~ =D
www.Sweetanime.com <-- This is another nice one that links to Veoh~ (Decent quality! YES! <3)
I never really liked
www.anime6.org. It was always super laggy and had a frustratingly annoying layout. >_<
And of course, as a last resort, there's always youtube. =/