A couple weeks ago a friend introduced me to Rock My Run, a website dedicated to sharing awesome running music with runners. It's free to register, and, upon registration, you get your first credit - for free! Songs less than an hour long are 1 credit, songs 1-2 hours long are 2 credits, etc. Also, you get a free credit each month!
My first credit went towards a mix called Dub Run. It kept me going during my 20-miler last weekend, so I guess it works.