GLXOSD (https://glxosd.nickguletskii.com/), licensed under the MIT license, is a monitoring tool for game FPS, system temperatures, and video card stats. It displays this information as an in-game overlay which can be toggled via a keyboard shortcut and customized via a lua config file.

The Steam Overlay offers similar functionality, but it only offers an FPS counter and nothing more. This tool would be helpful for analyzing frame timings and latency, validating overclocks, and so forth.

The latest release tarball/zip can be found here.

EDIT: Closed as WONTFIX because the project was discontinued; see below.