It's a really clean site that feels responsive (even from .nz) and isn't too cluttered -- great work!
A few bits of feedback: - When I was adding a new site to check, it wasn't immediately clear whether you support sending text messages to non-US mobiles. The only example provided is formatted US-style (1-xxx-xxx-xxxx). I put in my NZ mobile as 6421xxxxxxx and it did in fact work. I'd suggest adding a second example number formatted as e.g. +44xxxxxxxxx and perhaps a link to a page listing the countries you support. - Perhaps an option to send a second text message when the site comes back up? - The subject line of the email which is sent when the site comes back up is simply 'SiteCanary: accountname:'. It'd be good to have this be 'SiteCanary: sitename is back!' instead.