I have always believe in website uptime affecting our SEO ranking and reduces the return visitor. This is one of the very important criteria when you pick a webhosting and not the price. I am using this service from AreMySitesUp, as poor as many of you out there, I am using the free service with 5 domain name. This is fine with me, the only downside for the free account is that this website will delete inactive users, because I do not need to check the website for my status, I tend not to login to the website.
Anyhow, I think it is a nice service to share. Let me know if you use other service for monitoring your website uptime.