Check out SilverScream.net, where you can readily watch free black and white horror classics.
1. Frankenstein (1910)
2. Freaks (1932)
3. Doctor X (1932)
Fullhorror.com also has some good classic picks as well as newer modern horror movies selections.
Frightpix.com is a new site that has some really cool, diverse, mostly cult film selections you can watch for free online. They are newer movies, but I figured I would include them in this post, because there are movies you won't find anywhere else on this site and they support a ton of diverse devices! They are a pretty good Troma site!
You can check out which devices are supported for use with this site here.
There are a ton of supported devices such as Amazon Kindle, Apple iPhone, Apple iPad, Roku, XBox 360, and more!