You know, I'd like to ask a related question.
A while back, I gave a link to the Humble Bundle. For those not familiar, it's a website where you can buy independently-developed video games at the price of your choosing. You also can choose a portion of your payment to go to charity.
One of the few things I've paid for online was a comic that had a micropayment system attached to it. The artist asked people to read the comic and then pay what they felt like paying. It was a good comic, and I gave the artist a dollar.
I like the donation system. When I was in college, I used to host a party at my house every Friday evening. While I didn't supply alcohol, I did supply coffee (with cream and sugar) and snacks, and had a tip jar. I usually came out even without ever asking anyone to donate. I think voluntary payment works.
How many of you have paid for something that was payment-optional? What do you think about such systems of payment?