pen is a really neat new load balancer for TCP protocols like http and smtp. It's better than round-robin DNS because it keeps track of users and will try to send the same user back to the same server.
But a single pen server is still a single-point-of-failure, if your pen server goes down, your website goes down. So, use the very neato vrrp to help you build the ultimate cheap cluster
Now, use this with Amazon's EC2 and you've got a really awesome, and really cheap mega-cluster.
