I always go on these:
link (this site's great for speedy updates, but it's been known to lag and have formatting issues; however, it's also great for finding new anime to watch -- my first choice for finding new series)
link (I LOVE this site because, it doesn't have just episodes for streaming, but downloading too. An AMV maker's absolute best friend! Also, this site's not too bad on speedy updates too)
link (this is the site u go to when u JUST CAN'T find the series online. The selection of series on this site is so amazing! Problem is the pop-ups and ads, but if u get passed it, u have a perfect bron of online anime)
To a lesser extent, there is also:
link link and, when all else fails, I go to a google video search, but it'll probably give u the same linken I gave you.
^_^ I hope that helped~!