CPUSetter is a little Mac utility from Bryan Christianson that lets you set the number of active physical and logical CPU cores, as well as disabling hyperthreading on certain CPUs (e.g. quad-core i7), for various purposes, such as security issues, bug workarounds, power reduction, software license requirements, and pure experimentation.

Other features include built-in help, plus controls over process priority and limiting, as well as some very useful real-time displays of system/resource details — processes, priorities, and resource usage for CPU, GPU, RAM, disk, network, etc.

CPUSetter 1.5.0 is donationware for OS X 10.10 and up. The latest release adds a power usage graph and fixes a memory leak.