APPitic is an directory of apps for education by Apple Distinguished Educators (ADEs) to help you transform teaching and learning. These apps have been tested in a variety of different grade levels, instructional strategies and classroom settings.
Kindergarten.com is an app developer that has many, many apps (mostly free) with flash cards of various vocabulary classes. They also have a cool "Problem Solving" series of 3 apps called, What Rhymes? Which does not belong? and Which goes together. Check them out