Uptime Robot is a totally free website uptime monitor which checks up to 50 of your websites every 5 minutes and sends alerts by e-mail or SMS in case of any downtime.
After registration, you just need to input your website name and the URL, and decide to receive alerts by email or/and SMS, but the SMS service is not available for all countries or areas, for example, it is not available for China.
When the website is down, you will received an email alert like this:
And when the website is up, you will received an email alert like this:
Besides Uptime Robot, Was It Up is also a great website monitoring web app, and below are another top 5 free website uptime monitors:
2. Binary Canary
3. Are My Sites Up
You can check out DailyBlogTips for more details about these 5 free website uptime monitors.