Which airlines let children under 2 fly for free?

I know that the <A HREF="http://www.xjet.com/">regional airline ExpressJet</A> allows children under the age of 2 fly for free. Here is a link to their policy regarding<A HREF="http://www.xjet.com/menu/travelinfo/specialneeds/customerswithchildren.htm">flying with children</A>. I hope that helps!