Besides the obvious one (to / too / two, your / you're, we're / were and so on), the pack also offers some corrections for commonly misspelled words like espresso, figuratively and more. There are also a bunch of generic cross-out and insertion stickers, so you can really fix the horrible messages your friends send you. The app will set you back a buck on the iTunes Store, and it's probably the smartest use of stickers we've seen thus far.
Of course, just be aware that being a grammar snob is often the quickest way to make your friends ignore you completely. Still might be worth it, though.