Sudoku (by Finger Arts)

Last decade's puzzle craze is this decade's iPad freebie. A clean interface and endlessly generating puzzles -- along with a time-based bonus system -- will keep Sudoku fans entertained for hours in Finger Arts' app (there are a lot of games called Sudoku, so make sure you note the publisher). Harder levels unlock as you play. In case of uncertainty, players can also pencil in their possible guesses for squares. Ad support via small banners on the bottom keep this game free, and they're not overly intrusive. It all adds up to a perfect waiting-for-a-train diversion.