Someone asked if I could recommend a few starter apps. Below is a preliminary list (many of the apps have multiple uses):

Opposites: MyFirstApp Opposites, Baby Peekaboo Learn Opposites (Baby Cortex), Preschool Opposites (also good for descriptions)
Vocab: Peekaboo, English Baby, Fruit Find (older children), Playtime HD
Categories: Families 1 and 2, Sponge Words
Language Elicitation: Clicky Sticky, Birthday Party, Poke Me!, Dressing Up, Find Them All
Associations: Drag and Match, Clean Up 
Articulation/phonology: Minimal Pairs Lite, Blends (geared more towards phonics)
Description: See Touch Learn, Touch and Learn, Spot the Animals
Auditory Comprehension: Goodie Words
Multiple Uses: Baby, Peekaboo Pets, Mrs. Owl’s A to Z Lite
Reading: Rhyming Lite, Phonics Land, I Like Books, TimeReading