A lot of smartphone apps are just mobile translations of a standard computer program or website - useful, but they don't really take advantage of the strengths of mobile platforms beyond the interface. Here's an app that "gets" the way people use their phones, and tricks you into expanding your vocabulary. In a good way.
GRE Vocab Lock will give your phone a secondary unlock screen, which consists of a vocabulary word and two possible synonyms. Only one synonym is correct. Slide the word onto the right one to unlock your phone. If you get it wrong, the app makes a note of it, and will present you with the same word later. If you're unsure, slide the word circle up to the "d" icon and you'll see a brief definition. (The dictionary is populated for the Graduate Record Examination, but it should benefit SAT students or anyone who wants a more varied English vocabulary.)
The secondary lock screen appears after the standard Android slide/PIN/pattern unlock, so that your phone's security isn't compromised. Unfortunately, there's a set limit to the amount of words you get per day - 20 words for the free app, or you can upgrade to 50 words per day with a reasonable $1 in-app purchase. Why not give users the option to get a different word with every single unlock? The process is fast and simple enough that it would work well.
Even with that small reservation, Vocab Lock is a neat way to expand your word power, and it's cheaper than a word-a-day calendar.