Pingdom is probably the most well-known and used website uptime monitoring platform. They have all of the features you expect, plus some additional tools. What makes Pingdom unique is that their service not only tells you that your site is down, but also helps to diagnose exactly why it's down! They also monitor additional aspects such as page speed, and can even provide website user monitoring. Their pricing is a little higher than some others, but hey, you get what you pay for!