I would be interested what webapps do you know/use that allows you to monitor VPS. I would like to have the following metrics, CPU, Memory, Disk stats, Apache stats (hits/sec, free agents).
At first, check out munin. It's excellent and simple monitoring software. Unfortunately, it's not webapp.
Second, Pingdom is an excellent software for response time monitoring. It's free for only one target.
I am not sure whether there is any web services that could monitor Apache stats, but munin can do that.