Been using this little tool (GIGRIB) for a while at work to monitor uptime of our server. Its quite a neat program, which makes use of distributed networking. It works at bit like bittorrent (okay, so the similarities are slim…) but in order for your site to be monitored, you in turn monitor other sites. You can build up a buffer of time, so for example, my machine runs during the working day, but my site is monitored 24 hours. Its not just one to one either, there could be literally hundreds of people checking your site is up at any moment. Have a goosey.