There are different kind of clusters.
When the main site goes down, the backup site becomes active and so on. With this kind of cluster the linux-ha (google) project is helpful.
Hits are being distributed between the web servers so that they all share the load. In OpenBSD PF you can configure load balancing easily and it's probably quite easy in GNU/Linux too but I have no real experience with this.
Perhaps if you tell us what kind of cluster you want to build, we can help you more.