Volunteer distributed computing can trace its roots to the Great Internet Mersenne Prime Search (GIMPS), which was set up in 1996.  GIMPS was set up originally to discover larger and larger Mersenne primes (for more on Mersenne primes, read here) by leveraging a distributed computer network of volunteers. GIMPS has a computing throughput of 449 TFLOPS – nearly that of a supercomputer – which has enabled the network to discover the 16 largest Mersenne primes, including the largest prime number, currently known.  
It is amazing that GIMPS is still active and making discoveries over 20 years after founding, and volunteer distributed computing continues to provides researchers with access to computing power they otherwise would not be able to afford. In fact, the largest volunteer project, Folding@home, has over 8.7 million compute units in it’s network and as of September 2016 was more powerful than the most powerful supercomputer in the world.  BOINC, which is a volunteer and grid computing platform that supports multiple projects, has performance in the PetaFLOPS range and is the worlds largest computing grid. 
Bitcoin, the largest cryptocurrency by market cap, also leverages distributed computing – but not on a volunteer basis. Rather than donating computing power to scientific research, Bitcoin “miners” use their computing power to verify and validate transactions on the Bitcoin network. Every so often (to be honest, I’m not sure how this process works), one computer will successfully ‘solve’ a transaction and generate a Bitcoin for the device owner. Though comparing mining power to general computing power is a fuzzy science, some estimates claim that the computing power dedicated to the Bitcoin network is greater than the sum total of the 500 most powerful supercomputers in the world. 
Clearly, Bitcoin has successfully leveraged crowds. Though the number of miners fluctuates with the price of Bitcoin (at low prices, the energy cost of mining is greater than the potential earnings from mining), one company estimates that if the pace of mining growth continues, Bitcoin mining will consume all of the world’s energy by 2020.  It is an unlikely outcome, but it does demonstrate the rapid growth in mining. In fact, the Chinese government has even attempted to ban Bitcoin mining due to it’s energy consumption. 
Most Bitcoin mining was initially done on personal computers. In the same way individuals could donate computing resources to GIMPS or Folding@home, people would run mining software on their computer when they weren’t using it. Today mining occurs primarily on devices built singularly for that purpose.
Clearly people are willing to part with unused capacity of things they own, especially if they can generate returns as a result. This is the concept between many successful new companies like Airbnb and Turo. So the question becomes – what other things do people own that we can crowdsource excess capacity? Just like cars spend most of their time idle, cellphones live out most of their lives in our pockets. When phones are charging overnight or when we’re sitting at our desk they are underutilized and ripe for some sort of crowdsourcing platform – either voluntary or for profit. Expensive powertools are probably utilized even less than automobiles, but still it seems we go to Home Depot to buy a drill or electric screwdriver every time we have to assemble a piece of Ikea furniture (even though the instructions say no power tools).
 GIMPS History
 BOINC Homepage