Ajax has shown how viable it is to run client-side computations, while just downloading the raw data from a server. But Ajax is not restricted to doing fancy user interfaces.

It should easily be possible to use an iframe based ad to use the CPU power of page visitors to do some large-scale computations.

Can you imagine how much processing power Google could churn up by having its GMail users do some distributed computing? Or on YouTube. While the user is watching the video, a javascript does some calculations in the background.

By keeping data in a cookie, your calculations might even be able to survive page reloads. And if you're running a large ad network such as Googles', you might even be able to dect user inactivity. Update a cookie whenever the user comes onto an adsense page; if he didn't go on such a page for 30 minutes assume the user is idling and start computation. If he leaves open his web browser over night you'll get a lot of CPU cycles.

(Yes, I know that Google is supposedly not in desparate need for free CPU cycles...)