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.
"Lexico-Cognition is an App to develop language understanding, vocabulary building, cognitive, memory and auditory skills in a playful way. It was developed and tested by qualified speech therapists. "
Designed for 1st-2nd grade students, Murky Reef combines reading and math skills in the context of exploring a coral reef. This is the free version, though there is a more robust version available for $3.99. Students can review numbers, sight words, and vocabulary. There is some reporting built in.