Do you have a computer that runs 24/7 but is only actually used a portion of the day?
I have a desktop, laptop and server that are, for the most part, running all the time.
On these, I have installed the BOINC client which uses spare CPU cycles to do scientific research. By “donating” this CPU time, I’m helping to do medical research, search for extra-terrestrial life and find enormous prime numbers. Specifically the World Computing Grid, SETI@Home and PrimeGrid projects.
By installing BOINC and subscribing to projects, work units are downloaded and run at a very low priority, essentially allowing unrestricted use of the CPUs by foreground work and taking over the machine when it is idle.
Since my server is used to transcode media files at a relatively low priority, I have chosen to restrict BOINC to 80% of the cores/hyper-threads available which keeps it from slowing the machine down when in use.
For better security, install BOINC as a service by selecting an Advanced install. This will keep the clients running in the background when logged off.