I have been using Chill[1] periodically in lieu of music for a few months. It has the typical generators (airplane, beach, city, etc) but also blue, brown, pink, violet, and white noise tracks. I like brown noise; it has the low-frequency rumble of an airplane but the higher frequencies also block out ambient noise. Great for working in an office.

[1]: https://itunes.apple.com/us/app/chill/id421696351?mt=12