Despite being very money conscious right now I shelled out $20 on Friday for Flow. I can't remember where I got the link from but I'm damned grateful because it's great.
According to author Karlheinz Essl, Flow:
Generates an ever-changing and never repeating soundscape in real time that fills the space with flooding sounds that resemble - metaphorically - the timbres of water, fire, earth, and air.
I now tend to leave it running while I work and I find it really helps me to get into flow state and ignore distractions. I also find it quite restful in the evening before bed.