First time user from today. Really nice project. I have some feedback. Minor things, but I think would make the website slightly more appealing. You won't even need to spend more than 15 min to address several of these things if you feel they make sense.
I know what I'm about to share is kind of personal, though, but anyway I thought it might be interesting to share anyway. Take my feedback with a grain of salt. Feel free to disregard if you don't find it useful anyway.
Footer bg color
The bg color of the footer makes me think something is disabled. Every time I switch there (in the last couple of hours), I cannot "unsee it", is just like if there was a popup and the page was disabled. I know is a silly thing, but somehow, since I joined today I kind of feel that way because of the bg color of the footer (and body).
Using "#eee" instead of "#b1aaa0" would make it feel better. The bg color on "Documentation" and "Support" buttons might be too prominent, I would change to #ddd or other gray variant or remove them altogether.
Footer eating too much space
If you don't have many alerts, footer is eating a lot of space that would be better off empty in white. Footer could still be visible. I know this would require come CSS trickery, so if bg is changed it might not be as annoying (to me, at least; again, this is personal so it might just be me).
Buy Credits "hard" to find
I think it will also be interesting to add a "Buy Credits" or similar button at the top of the page, since it will be a 1-click thing for people to add more credit. Haven't been long enough to know if you send emails after some sort of threshold is crossed (e.g less than 10k credits, or things like you'll run out of credits in less than 2 weeks, you can buy more credit anytime...). I guess that's there already, but I still think a button like that is good to be present and easy to find in 1 click.
I mean, you can find it in 2 clicks, but 1 click is better for what gives you money :)
Minor bug on header's time remaining
If you update any of the monitors check (e.g from 10 min to 30 seconds), the computed time at the top does not get updated immediately, you need to refresh the page.
Send Test Notification
I had my first time user experience today. I missed to "Send Test Notification" functionality (that uptimerobot has) in order to assess I could somehow trust this page and that notifications were getting through on what I configured.
I ended up forcing my website to fail just to see that notifications were working, but it took more work on my end. It might be handy to have this when somebody just sets up the account the first time, or when it creates a new alert. Just a thought.
Other than those details, really cool project. Really well executed.