This app was mentioned in 2 comments, with an average of 4.00 upvotes
FieldMove Clino is an app that acts as a compass and clinometer to measure dip/strike directions. It also uses GPS and can plot the measurements directly on a map. I haven't used it for actual data capture (I rather still use an actual manual compass), however it is fun to use on excursions and field trips.
FieldMove is a map-based digital field mapping app for geological data capture. It is fairly expensive, but is actually a full blown mapping software that works very well with tablets.
For more apps just go to your preferred app store and search for "geology", or look at the "related"-tab on the FieldMove store page.
FieldMove Clino is awesome for field work.