This free app from McGraw Hill is great for elementary school students to work on building their vocabulary. Students can either play on their own or with a partner. Bluster works on rhyming, prefixes & suffixes, synonyms, homophones, adjectives, and over 800 vocabulary words.
This is a great app for elementary aged students. They can practice telling a story by using the visual prompts. It records their narration into the app to play back later.
This team of teachers from the National Teacher's Academy did an excellent presentation at the EdTechTeacher iPad Summit. This site lists some of their favorite tools, apps, and sites.