There are absolutely tons of resources here: http://www.squidoo.com/50statesnotebook
State flag coloring pages here: http://www.apples4theteacher.com/usa-states/state-flags.html
We've enjoyed this book: http://www.amazon.com/Smart-About-Fifty-States-History/dp/0448431319/ref=sr_1_1?s=books&ie=UTF8&qid=1319525142&sr=1-1
We have a puzzle that has the pieces cut on the state lines similar to this one: http://www.toysrus.com/product/index.jsp?productId=2326654
Scrambled States of America is fun (books and video, don't have the game yet, ordering it soon, I think).
Stack the States, though, is by far our favorite. DS (just turned 6) has learned all 50 states as well as plenty of random facts about each state just by playing a few minutes a day.