ArsTechnica has a nice article about the suitability of the Amazon cloud for High Performance Computing. I had linked to two previous stories about HPC clusters built on Amazon EC2. In short, Amazon EC2 tends to be better suited to embarrassingly parallel code where there is not much inter-node communication. This leaves out applications like finite-element simulations and molecular dynamics simulations. They mention R Systems which specialize in renting out Infiniband-connected clusters: Infiniband offers low latency, high speed connections.