That's a good question. Uptime Robot is launched on January 2010 and it is totally free since then.
It all started with the desire to create something free + useful for the web community. Within time, the service grew so much, we spent lots of hours improving it and learned a lot.
Update Jan 13 2015: We have introduced a Pro Plan today. And, the Free Plan is still here to stay :).
The main monitoring location is in Dallas-USA and there are other nodes in other countries/continents to verify down statuses.
All the locations and IPs used can be found in the Locations & IPs page.
Here are short details about monitor types:
- http(s): that's perfect for website monitoring. The service regularly sends requests (which are the same as if a visitor is browsing your website) to the URL and decides if it is up or down depending on the HTTP statuses returned from the website (200-success, 404-not found, etc.)
- ping: this is good for monitoring a server. Ping (ICMP) requests are sent and up/down status is decided according to the "if responses are received or not". Ping is not a good fit for monitoring websites as a website (its IP) can respond to ping requests while it is down (which means that the site is down but the server hosting the site is up)
- keyword: checks if a keyword exists or not exists in a web page
- port: good for monitoring services like smtp, dns, pop as all these services run from a specific port and Uptime Robot decides their statuses if they respond to the requests or not.
Uptime Robot currently uses the free email-to-SMS gateways for the providers that are told to support it.
It is almost impossible to debug any issues regarding the deliveries as Uptime Robot simply sends an e-mail with a specific format and that is all, no responses returned.
However, we have added a pro-SMS-gateway option (priced 50 credits/$15 or 100 credits/$25) which shall work for everyone.