He mentioned Pandora and that was the first service that came to mind for me. I wanted a way to donate to them, but couldn't find it (frankly, I didn't look very hard either, I'm sure if I sent an e-mail they'd find a way for me to send money).
A year ago I purchased a "registered" copy of 7-zip for $20 some odd dollars because I wanted to support the development of that application. It afforded me e-mail support or something like that, but I have never bothered the developer. I just loved the application.
I don't command a six figure income. I'm also the sole financial provider for a family of four. I recognize value when I see it. It is a sort of "tip" to the people who I'm reaping benefits from and I have no problem showing my gratitude monetarily.
So ... another way to look at this is ... you have a winning product if people are asking for ways to pay you for what you're giving away.