Probably the best way to monetize an idle CPU is to leave it idle, so that it uses less electricity. If electricity isn't a concern, and you have a lot of CPU

Probably the best way to monetize an idle CPU is to leave it idle, so that it uses less electricity. If electricity isn't a concern, and you have a lot of CPU power, perhaps you can get into the cloud business. While AWS and Azure have come to dominate the market, there are still smaller providers (usually called "VPS" providers) that are doing well for themselves.