Unixshell I/O Wait
This is a Munin graph of helium.anthrocon.org under a normal load. Another VPS was killing the disk which caused one of the cores to pretty much block on all I/O operations. Doing a simple ls took in excess of 5 seconds. Trying to serve up a single webpage took over a minute.
When I opened a troubleticket with Unixshell, they actually said, "gee, looks okay to us", and closed the ticket, without fixing the underlying issue. At that point, I took my business elsewhere.