It must have been back around 1999 when I first heard of distributed computing... where people volunteer their computers to run small pieces of a larger research effort. I helped with a cancer research effort for years through UD... United Devices. They disappeared.
A new distributed computing model was developed called BOINC into which any number of different projects could be plugged into. BOINC can run as many projects as you have CPU cores and you can switch projects whenever you want. I ran two for years on my old dual core PC... one medical, the other on cosmology. BOINC can also harness the immense power of GPUs.
There are any number of projects you can participate in... and a list can be found here...
(edited and moved from the environment forum)