There are freeware programs that do this that run from your PC, but they are pointless if your computer goes into suspend / shuts down / your BB dies etc...
You can get external companies to monitor it for you. I've used Alertra in the past, and it is very good - however, you do get what you pay for.
For a low cost alternative, you could try siteuptime.com, or even have a stab at using something like Nagios that you install on a server to check for you - obviously, using a separate hosting company to host this, in a different data center, or even country!