[sigil]

This isn't a new idea, although we have yet to see someone execute it well. Plura Processing tried it back in 2008. Here's their Show HN:

http://news.ycombinator.com/item?id=347359

Plura ran into trouble because they built a pay-for-grid computing platform where the nodes were unsuspecting users:

http://pluraprocessing.wordpress.com/2009/08/24/our-response...

Personally I'd like to see a SETI@Home or Folding@Home in the browser first.