LetMeTalk: this is a FREE AAC application! It is in both Spanish and English, and you can customize the categories the way you want within minutes. They have all the pictures you can think of, and you can take your own pictures to add new ones.
PECS phase III: This is a great first choice application, as it is user friendly and easy to add pictures and move them around. You can add 1-8 choices of pictures. It costs somewhere between $5-10.
My Play Home: Absolutely favorite therapy game application. This is good for all ages, and can be used to: follow instructions, answer wh questions, teach vocabulary for everyday items, used as a reinforcer, taking turns, etc. About $5-10.
Yes No: This is a simple application to teach Yes and No, but it works great! About $2.