If you know much about Tor, you know that all connections come from localhost. Even though it’s old news (I first heard about this a year ago) it has come up in the news recently.
It reminded me of a proof of concept I wrote for rate limiting hidden services, or alternatively, any service where you can’t distinguish users. Basically, you have them prove they did some amount of work (and therefore spent a certain amount of time between requests)
Factoring a semiprime, for example. It’s slow, which is why it is the basis of RSA encryption. More on that in the near future 😉
Update (Feb 15): There’s now another version of this concept available, which operates more similarly to bitcoin.