Trainyard may be a simple iPhone game, but it's one of my latest addictions. Folks who are nuts about logic and puzzle games—as well as those who adore Flight Control or ColorBind—are very likely to love it, too.
The game's currently on sale for a buck, but normally priced at $1.99. If you're not absolutely certain that you'll like it, you can always try Trainyard Express which is free, but contains far fewer puzzles. [iTunes]