Free trending service

[MachMetrics]

It's been a few years since the list of 3 above was posted, and it's a bit outdated.

Here's the most recent list of speed monitoring services - who still are not free since as Patrick said it costs lots of server resources running these tests on a recurring basis:
https://speedcurve.com/
https://www.pingdom.com/product/page-speed
https://gtmetrix.com/pro/
https://calibreapp.com/
https://loadfocus.com/
https://www.machmetrics.com/

I hope you don't mind that I included ourselves on there - MachMetrics has been around for more than a year and are currently providing automated speed tests to over 400 users, from 5 regions, various devices, multiple times per day - all powered by the wonderful WPT engine