TCEA (Texas Computer Education Association) helps by testing and recommending great apps for the classroom. They regularly try out new apps and list those that make the grade in the shared document available here. Apps are categorized into 46 different subject areas and 14 different personal use areas. Apps that are free are listed in white. Make sure you check out the rest of their site for some great resources.
http://appsineducation.blogspot.com/2012/02/1000-recommended-apps-sorted-by-subject.html?spref=tw