I am also in the process of learning CSS. Like Freestyler said, w3schools.com is one of the best. Here are a few other good ones.
8 Cheap And Easy Ways To Learn CSS
This site has some great resources. They also offer some free online classes. I'm not sure, but I think that the CSS one is a five day class.
The Complete CSS Tutorial
This is another goood one.
This one is a little on the basic side, but it is still great for starting out.
Sitepoint CSS Reference
I haven't read all of this one, but what I have read of it so far has been great.
Hope that this is some help to you. Good luck!!!