A little more detailed answer...
A proof-of-work plugin exists for WordPress and probably others that is based on hashcash, so the same idea. http://wordpress-plugins.feifei.us/hashcash/
You could extend this idea to Bitcoin mining. Unfortunately not using WebGL though, as OpenGL doesn't include the programmability that CUDA or OpenCL do to make things fast and provide for good two-way communication, as far as I know.
I'm sure someone's toyed around with making a js snippet that has all of the website visitors perform a bit of CPU-mining while they are visiting or just for commenting. For a high-traffic site it would certainly be workable if the visitors are up for it. It would probably even be less intrusive than banner ads or CAPTCHAs.