Both of the suggestions already made are good. Ravelry.com is a social site for fiber enthusiasts that also has a large catalog of free and to pay patterns.
Knitting Pattern Central is an adequate resource. I find a lot of broken links when I go through that site looking for a pattern.
You can also enter your parameters into your favorite search engine to see what comes up. Unfortunately, a lot of knitting pattern lists were put out there a long time ago and have been left to disintegrate as their links have disappeared. But there are several yarn manufacturer, craft catalogs and craft magazine sites that offer free patterns also.
You have a lot of options for finding free knitting patterns on-line.