Most every airline is part of a larger alliance (except for discount carriers like Southwest, JetBlue, etc). You can find out who these partners are from the website of any airline. For example, the following link is from NW's website.
http://www.nwa.com/freqfly/earn/airfl.shtml
I'm sure you can easily find similar pages from United and American's website.
Remember that each airline has its own frequent flier program (like AAdvantage for American and Mileage Plus for United), while also being part of a larger alliance (OneWorld for American and Star Alliance for United).