Please review my app: Was it up? - dead simple monitoring
xinsight: I wrote a similar tool for monitoring my own servers. Besides being "up", I find that it's important to track slow responses (>10 secs) and to differentiate between DNS problems and web server problems.(I also find that it's useful to check for load, disk space, mysqld is responsive, etc. but that's would take away from the elegance and simplicity of this tool.)