I really like Relax Lite (it's got an orange icon with a blue smiling face on it). It's a free app that was recommended to me. It leads you through a guided meditation that lasts about ten minutes. This is supposed to be a meditation that can be used either during the day to refresh/ground yourself or at night to help you go to sleep. I used this a lot at night while I was working at my old job because I had a really hard time letting go of the events from the day. The narrator has a very calm voice and does a good job of leading you away from your own thoughts, centering you instead on your body so you can relax it.