Being a webmaster or blogger, it is not easy to monitor a site’s downtime. There are a few services who offers these thing, like pingdom. However, these services are not fully functional until we pay some bucks.
Their is a new site tagBeep launched in this segment to monitor server uptime and users will be alerted if any downtime found. For free one can monitor upto 50 sites and server. If any downtime found, user will get email and/or SMS alert (Upto 20 SMS for free during Beta) on their mobile. The site will be monitor from two Geo Location one from USA and another from EU.
Their are a lot of customization which is enough for most of us.
Their is also a desktop application which will alert if you will be online from your PC.