Android: It's hard to improve upon a simple alarm clock app, but Timely has done just that with a beautiful, customisable app that syncs alarms across devices.
Timely eschews the flat design that's been in vogue lately for an ethereal, floating interface. Where it stands out is both in creating new alarms (done with a simple gesture), as well as alarm sync. From your phone, you can decide which alarms are active on your tablet or other devices (and vice versa). When the alarms go off, dismissing them on one will deactivate them on all. This could be useful for creating a surround sound-style alarm without creating a horrible labyrinth of alarms that all need to be turned off individually. The app is free, but certain customisation options can be unlocked for $2.99.