I know a lot of folks that raise rats for pets have this link but I've used it a lot to help me manage our breeder/feeder rat colony and I consider it one of the best sites out there. Becky gave it to me ages ago and I just wanted to share it with some of our newer rat keepers.
http://www.ratguide.com/
It's very well laid out, very easy to navigate and has an excellent search engine.