We are thinking of entering GRE iPhone/iPad market and I kept an eye on what's going on there for awhile. I can't recommend any particular app as I didn't test them well but it seems that different types of vocabularies are very popular, such as
GRE Exam Vocabulary Flashcards and Reference Guide"
"GRE and GMAT Vocabulary builder"
"Smart Vocab (GRE)"
"PowerVocab GMAT and GRE" is a new app.
Some other providers offer questions and I didn't try them. At the same time many of them have GMAT apps too and are not popular at all. So, I would recommend you to search for "GRE" in the appstore and try apps only within top1-10. Very rarely you can find a good app that has lower position than 10.
Month ago I tested one XYZ (not GRE) Vocabulary app that claims all words were manually picked by XYZ experts and tutors. When I ran an analyzer through all official tests, some words were not even mentioned in official tests and some had statistical occurrence as any other 50K+ English words. I don't claim that all apps are bad but some really are. So, approach them with a grain of salt. Reading reviews is useful but for many reasons they could be biased.
Anyway, as I mentioned at the beginning, GRE vocabularies should be good.
If you have other questions related iPhone/iPad apps, feel free to pm me.
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