When it comes to hosting my websites I like to spread them over multiple web hosting companies. It does improve my odds of all the sites going down at the same time, but tracking their uptime becomes an issue. There are website uptime monitoring services one too many, most of them have a catch either they want you to place images with backlinks to their website or something more silly.
A tried and tested free uptime monitoring service I use is http://uptime.alal.com/uptime/ , originally written in TCL its now run by OpenACS . To track a website just add a URL to text file or HTML page on your website which contains a single word “success” in it. And choose whether you want to get email when this website down or just check periodically every few hours. Compared to other website uptime monitoring services this one does only one thing and does it well too.