The following quote is from wikipedia under the heading
"PlayStation 3 cluster"
The NCSA had already built a cluster based on the PlayStation 2. Terra Soft Solutions has a version of Yellow Dog Linux for the PlayStation 3, and sells PS3s with Linux pre-installed, in single units, and 8 and 32 node clusters. In addition, RapidMind is pushing their stream programming package for the PS3.
On January 3, 2007, Dr. Frank Mueller, Associate Professor of Computer Science at NCSU, clustered 8 PS3s. Mueller commented that the 256 MB of system RAM is a limitation for this particular application, and is considering attempting to retrofit more RAM. Software includes: Fedora Core 5 Linux ppc64, MPICH2, OpenMP v2.5, GNU Compiler Collection and CellSDK 1.1.
In Summer 2007, Dr. Gaurav Khanna, a professor in the Physics Department of the University of Massachusetts Dartmouth independently built a message-passing based cluster using 8 PS3s running Fedora Linux. This cluster was built with support from Sony Computer Entertainment was the first such cluster that generated published scientific results. Dubbed as the "PS3 Gravity Grid", this PS3 cluster performs astrophysical simulations of large supermassive black holes capturing smaller compact objects . Khanna claims that the cluster's performance exceeds that of a 100+ Intel Xeon core based traditional Linux cluster on his simulations. The PS3 Gravity Grid gathered significant media attention through 2007  , 2008 , 2009    and 2010  . Khanna also created a DIY website  on how to build such clusters, accessible to the general public. end quote.
If you are interested in building your own cluster I would recommend finding PS3s on someplace like Ebay and specify that they must have been built before March 2010 when the capacity to use multiple operating systems was disabled by Sony.
begin quote from same wikipedia site listed above:
On March 22, 2007, SCE and Stanford University expanded the Folding@home project to the PS3. Along with thousands of PCs already joined over the Internet, PS3 owners are able to lend the computing power of their game systems to the study of improper protein folding and associated diseases, such as Alzheimer's, Parkinson's, Huntington's, cystic fibrosis, and several forms of cancer. The software was included as part of the 1.6 firmware update (March 22, 2007), and can be set to run manually or automatically when the PS3 is idle through the Cross Media Bar. The processed information is then sent back to project's central servers over the Internet. Processing power from PS3 users is greatly contributing to the Folding@home project, and PS3s have overtaken all other participating operating systems in teraflops contributed. As of April 23, 2007, more than 250,000 PS3 owners have allowed the Folding@home software to be run on their systems, averaging over 400 teraFLOPS and peaking at over 700. By comparison, the world's most powerful supercomputer as of April 2010, the Cray Jaguar has a peak performance of 1.75 petaFLOPS, or 1750 teraFLOPS. The latest report stated that Folding@Home has passed the 1PF (petaFLOPS) mark, of which 800TF(teraFLOPS) are supplied by PlayStation 3 clients.
The Computational Biochemistry and Biophysics Lab in Barcelona has launched a distributed computing project called PS3GRID. This project is expected to run sixteen times faster than an equivalent project on a standard PC. Like most distributed computing projects, it is designed to run only when the computer is idle.
eHiTS Lightning is the first virtual screening and molecular docking software for the PS3. It was released by SimBioSys . as reported by Bio-IT World in July 2008. This application runs up to 30x faster on a single PS3 than on a regular single CPU PC, and it also runs on PS3 clusters, achieving screening of huge chemical compound libraries in a matter of hours or days rather than weeks, which used to be the standard expectation. end quote.
So, the capacity for PS3 clusters potentially to be used for good in helping mankind is potentially almost limitless. So, if you have a good use to put a PS3 cluster to---Go for it!