I'm building something which could uncharitably be described as "an open proxy providing attack amplification as a service", but which I'd rather describe as "a core part of tools like RSS feed readers": ask it to monitor a URL, and it'll check periodically to see if it has changed and tell you so. I'd like to make it a publicly available service so people can more easily build new tools for noticing when web pages change. What are some ways I can encourage good applications and discourage abuse?

edit to add: My progress so far is at https://github.com/jameysharp/gawkurl/