What are some alternatives?
When comparing iStat Menus and Core Temp, you can also consider the following products
SpeedFan - Hardware monitor for Windows that can access digital temperature sensors located on several 2-wire SMBus Serial Bus. Can access voltages and fan speeds and control fan speeds. Includes technical articles and docs.
Open Hardware Monitor - Monitors temperature sensors, fan speeds, voltages, load and clock speeds, with optional graph.
MenuMeters for El Capitan - MenuMeters for OS X El Capitan 10.11 (and later)
Sidebar Diagnostics - SidebarDiagnostics - A simple sidebar for Windows desktop that displays hardware diagnostic information.
Real Temp - Real Temp is a temperature monitoring program designed for all Intel single Core, Dual Core, Quad...
CPU-Z - CPU-Z is a freeware that gathers information on some of the main devices of your system : Processor name and number, codename, process, package, cache levels.