SAT Vocabulary This great free app has 4922 words and it lets you mark off ones that you have already learned. You can view words in alphabetical or random order. I would like a counter on this app to see how many words I've learned.
Vocab Builder This free app has a built-in dictionary and thousands of words. It has some stats to track your progress. Downside is that the words aren't used in a sentence - you just get the definition of them.
PowerVocab Vocabulary Word App This free app is the best as far as different ways you can learn the words - but it doesn't have as many words as the other apps.
Cambridge Academic Content This is the ultimate in vocabulary in all fields, math, science, the arts, social studies, history, etc. For this amazing collection of knowledge you have to pay US$11.99 - but it's worth it.
CueBrain! learn spanish + more For US$3.99 you get unique and interactive ways of building vocabulary (cue cards, drum machine, lots of stats) It has a lot of languages!