I just realized that the Instapaper subscription gave me exactly what I have wanted from a lot of other software developers. I have actually emailed people who maintain free web services to ask them how I could give them money. If someone makes something that I use and appreciate, I want to be able to support that effort and show that appreciation, and aside from evangelizing, helping to fund further development is the best way I can think to do so.
The Instapaper subscription does just that. Marco even says so in the About Subscriptions section:
What are the benefits?
Right now? Almost nothing, except knowing that you are supporting the Instapaper service’s operation and future feature development.
I’m going to start including a link to that page when I email other people that make free software, and say, “This is what I want. Set something up so I can give you a little money every month, and I’ll bet other people will, too.”