Does anyone have any experience with HashCash or alikes? http://en.wikipedia.org/wiki/Hashcash http://en.wikipedia.org/wiki/Hashcash Obvious problem is that in

Does anyone have any experience with HashCash or alikes? http://en.wikipedia.org/wiki/Hashcash http://en.wikipedia.org/wiki/Hashcash Obvious problem is that in case of directed attacks spammers (or other malicious persons) would use native code or even GPUs to perform computations, while ordinary users would be limited by slower JavaScript implementations. It would help (but not solve the problem completely), though, if someone would show us some non-trivial code, that performs computation in background (without any noticeable impact on page interactivity), that'll finish in some minimal time required to write a meaningful comment or fill out a form.