There has been some talk of Website performance monitoring tools and services on stackoverflow, however, they seem fairly expensive for what they actually do. Are there any good opensource libraries for automating checking/monitoring the availability of a website?
If you just want to know if your server is serving out content or not, take a look at Montastic. I use it, and am pleased. Plus its free!
It will ping your site periodically, and if it doesn't get a 200 status, it lets you know.