SPQR (App Store link) is the swiss army knife of iOS apps for Latinists. It has a plethora of features that make it worth the price tag for any serious student of Latin. In fact, there are so many features, that this post will only cover the core ones in the full name: SPQR Latin Dictionary and Reader.
SPQR includes Lewis and Short. This seems pretty much the same as the Lewis and Short in Logeion (review of Logeion for iOS here). But SPQR also includes an English-to-Latin dictionary. Here are screenshots of the SPQR dictionaries.