Nagios for monitoring everything (including virtual hosts)
They said it was hard to do; it's not.
For some time I've been using Nagios to monitor my own server and that of some clients. One of these machines has a batch of virtual web servers. Nagios' documentation doesn't say much about monitoring virtual hosts, and a web search only turned up people using extreme copy-and-paste to implement this (even a script to automate the redundancy), so I came up with a better way.