About once a year I get an email from someone who wants to start a company selling CPU time from idle computers, typically by running a screensaver or background job on public computers. Back in 2000 I co-founded Popular Power, a company to do just that, which failed about 15 months later. Here's what I tell the people who write me looking for advice.
The primary challenge is finding customers. It is difficult to convince any company to risk sending their software and data out to untrusted third parties. There are no good technical measures for protecting intellectual property on untrusted computers. In addition, scavenged cycles are an awkward mode of computation and potential customers have to port their code before they can even try it out. A company's primary value proposition is price, but you're competing with the ever-decreasing cost of computer time. And the market is mostly $200,000+ enterprise sales. It always takes 12–18 months to close a contract at that scale, that's a long time wait for a cash-poor startup.
A couple of things have changed in the past ten years for the better. Cloud computing is now mainstream, there's no need to explain the concept. And distributed computing has improved with paradigms like MapReduce. Computers have gotten faster, although that doesn't specifically favor scavenged computing. Unfortunately bandwidth has not gotten significantly cheaper so data transfer is still a barrier.
Fate has not been kind to this market in the last ten years: see my 2007 retrospective. Since then United Devices merged with Univa and is still operating, although presumably never worth anything like the $45M in venture investment it raised. Data Synapse got bought by TIBC for a rumored $28M, more than zero but not the $200m+ value of a real success.
I'm glad people keep trying to make this business work, but it's a tough road. In the meantime free projects like BOINC are having some success providing low cost computation for public research projects. And of course the botnets are owning the Internet.