Bit of shameless advertising, my brother-in-law has his own small game company and they've just released their first game (iOS/Android), one of those free to download in-app purchase jobbies. I played the beta a fair bit and found it a decent way to kill time on the train or sat out shopping with the wife so I'd say it's worth a download:
iOS - Apple App Store link
Android - Google Play Store link
Also, if you like it, tell a friend!