I am a bit of a trash picker that happens to live near a fairly well off development. I'm known to drive the streets, the day before trash collection, picking up unwanted computers. And I've found some real doozies too.
The result of my efforts is one of the slickest single image cluster, compile farms, this side of NASA.
I love Linux and I compile source code more often than most people change their socks. To cut down on the compile time, I use my cluster to spread the compile over several nodes. When I get into a big source code build, I can run 10 nodes with one master.
The master is the one I begin my compile on and as more and more threads are called up by the make process, they get migrated out over the network to the least active node. When the compile is completed on any node, the binary is sent back to the master node. So, for instance, instead of one machine compiling a 100 meg source tree, I begin the compile with "make -j 30..."... each new make get's migrated off the master... really, really slick. It makes the network sing. Some day, I'll find enough 1gig nics and make it even faster
I run Gentoo, a MOSIX kernel, a twelve port switch (not hub) and ccache on each node. I was very careful about the nics I use, as I wanted to make certain I could atleast get duplex 100base-t transfers. Compile times are breath taking. I can compile a fairly complicated kernel (lots of drivers and options) in under a few minutes on the worst day. Smaller programs are pretty much a few eye blinks and they are done and ready to use.
MOSIX is available for free, educational use. Just google MOSIX. It comes as a patch that you apply to a vanilla kernel source and it also provides all the tools you'll need to get your cluster up and running.
How this all plays on ARM, I don't know but it would be fun. It would use a lot less power than what I have when running the whole cluster. it's something I'm looking to try with a few of the next gen RaspberryPi's. It'd be cool to see a multicore, high clock rate RP someday, That would make a nice cluster regardless.