I highly recommend Penguin Computing (Score 3, Informative)

We recently puchased a 13 node dual Opteron cluster from Penguin Computing after evaluating clusters from them, Dell, IBM, HP and single memory image machines from SGI. Penguin provided a solution with the best price/performance as well as ease of use. They let us benchmark our codes on some of their test clusters to determine wheteher the Opteron or Xeon based clusters would be better suited to us. My favorite aspect of their system besides price was the plug-and-play setup. The cluster was shipped fully assembled, configured, and tested to our site. All we had to do was roll it out of the crate, plug in the power and network connections, configure the network settings in the OS, and start running our simulations. All of the solutions from the other vendors would have required significant setup time on our behalf unless we spent a large amount of money for the services. I also really like the Scyld operating system that was included in the cluster. It makes the cluster work almost like a single image memory machine. Scyld on the compute nodes is setup to download the kernel image and necessary libraries from the master node at boot-up, so any changes made to the master node automatically propagate to the compute nodes. After several months of running simulations, the cluster has not given us any problems. It has been very reliable (never needed a reboot). Their technical support has been very responsive about ansering questions we had with the initial startup.