Another really good alternative would be pingnik.com
2019 jan 07 I just received an email from Pingdom announcing that they will no longer have free accounts. I've been using them for years, one ping per client site. I will now move to UptimeRobot (which i've also been using for other projects). Here is their statement:
"To ensure the best possible product experience for SolarWinds® Pingdom® users, and to focus our resources and investment on this next phase of our product development, we will no longer offer our free plans as of February 6, 2019. To continue enjoying the benefits of Pingdom, all free users will need to sign up for a paid subscription plan option by this date."
I'm not leaving a rating because i have not tried their paid plan.
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