This is possibly the highest rated meditation app around with 4.9 stars on the play store. Personally, I have found the app to extremely useful and in just a month of ~10 minutes a day have really begun to feel a tangible change and reduced suffering in many of my daily experiences. It also definitely offers something noticeably different than some of the meditation apps I've used.
I thought it was a lovely touch that they offer:
"If you would like to use this app but truly
cannot afford it, please email us, and we will
provide you with a free account."
A subscription is normally £6.49 a month (subscribe before 31/1), however they also offer a money back guarantee:
"GUARANTEE: If you complete the 50-day course in less than 90 days
and don’t find it valuable, we want you to have your money back. Please
email us, and we will refund your money."
Regardless - the first 5 meditations and a few lessons are also completely free (no emailing or subscription required).