The Department of Energy website is an absolute treasure of knowledge concerning anything you could want to know about energy in this country. Plus they do a good job of making the information understandable.
You can get a quick definition of what a renewable portfolio standard is and a display of which states have them here: DOE RPSs
Keep in mind that this information is from 2007, which means it does not include the RPS that Michigan enacted in late 2008. Here is Michigan's RPS information:
Organization administering RPS: Michigan Public Service Commission.