MonitorServerUptime is a monitoring solution for small websites
Large websites (like DownloadSquad or any other major blog/site) have full-time sysadmins, dedicated hosting facilities, and a ton of fail-safe mechanisms to ensure that they're always up and available.
However, if you have a small site that's running off of a shared host you may sometimes be surprised to discover that it's down. That rarely happens, but when it does happen you usually discover it yourself -- or a friend tells you. By then, there's no telling how many visitors you may have lost (dozens, usually, but that's a lot of visitors for a small, personal site).
MonitorServerUptime is an inexpensive service (US $3 per URL/server per month) that promises to send you an email or SMS alert (worldwide) whenever your site goes down.
What you do from there is really up to you. When you're using a shared server and need to rely on your host's support services, though, there's often not much that you can do (outside of sending an email and hoping for a quick and effective reply). But still, being one of the first to know that your own website is down does help.
Know of a similar service which you think is a better deal? Tell us!