What are some alternatives?
When comparing SpeedFan and GKrellM, you can also consider the following products
Open Hardware Monitor - Monitors temperature sensors, fan speeds, voltages, load and clock speeds, with optional graph.
Conky - Latest commit 262a292 on Dec 7, 2017 brndnmtthws Add missing build dep. Conky is a free, light-weight system monitor for X, that displays any kind of information on your desktop.
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.
Rainmeter - Rainmeter is a desktop customization platform.
iStat Menus - "An advanced Mac system monitor for your menubar."
glances - Glances is a cross-platform system monitoring tool written in Python. Written in Python, Glances will run on almost any plaftorm : GNU/Linux, FreeBSD, OS X and Windows.