Even aside from craft store apps that give you coupons, there are tons of apps out there for knitters and crocheters. These are my personal favorites.
Knit: a row/stitch counter that can keep up with multiple projects at once. The developer doesn’t speak English that well, but the cuteness of the app makes up for that.
Wooly: If Ravelry had developed an app, it would be like this. You can add projects, add stash items, and queue patterns, you can add photos (which is great if you’ve experienced the frustration of trying to add photos to Ravelry on a mobile device), check your messages, and more. Super handy.