You can use Uptime Robot, it offers a very good free layer (up to 50 sites free).
You can configure your sites (they are called monitors) in many ways. A simple GET (your case), ping and more. In case you want to have an idea, the screen to add a monitor look like this:
Image may be NSFW.
Clik here to view.
You can also configure your email to receive alerts, it has more options but I haven't explored which one of them are on the free tier, but in the other options you can find SMS, slack, hipchat and more.
It has a very good dashboard (I never use it though, I just get the email notifications) where you can see the status of the health checks and the type of error if it's the case.
Image may be NSFW.
Clik here to view.
Image may be NSFW.
Clik here to view.
The only downside on the free tier is the five minutes interval, I would prefer to use a two or three minutes interval but most of the sites don't need one second accuracy on the health check anyway.
