Sleep Blaster for iPhone wakes you up when you reach your destination
I can't count the number of times I've slept through a stop, and Sleep Blaster would have been a clever way to prevent that from happening. It might not work on the subway, though, where reception is harder to come by. Because an iPhone GPS is a fickle thing, and not always entirely precise, it's useful that Sleep Blaster lets you draw a line on the map, instead of just a single point. It also has a feature I've wished many of my alarm clocks had: you can yell at it to turn it off.
That might not be great if you're the type who intentionally puts the alarm clock across the room to keep from shutting it off before you wake up, and you probably don't want to combine the yelling with riding public transportation, but it seems pretty cool anyway.
A few other cool features of Sleep Blaster: it can generate ocean wave sounds to put you to sleep, and it has a Dynamite Mode that wakes you up with an extra-loud boom. If you don't like being jarred awake by an explosion, just pick a nice MP3 from your iPod instead. Also, it solves the problem of accidentally switching AM and PM, by asking you to confirm your alarm time if it seems like you made a mistake. God, I wish my college alarm clock had included that feature.