I have one that used to be for running simulations and xcode compiling. The shift to multi-core 64-bit stuff has somewhat retired it, so now it's mostly for parallel/cluster computing experiments. Probably the most useful task it has now is video encoding. My house is electric heat so I usually fire it up during the winter as a slightly more productive way to turn electricity into heat. For a while I played around with xgrid, but I think that has been dropped by apple for some reason. It would be nice to see more stuff written to use multiple computers vs. the now common multi-threaded software.
For non-programmer usage, you could do a load-balanced webserver/media server/home server. Cost wise you could launch your own retro Yahoo directory & search using a FW400 enclosure for storage(PATA internal HDD of the mini would be too old & unreliable or expensive to use an SSD).
At the moment I dusted off a old ITX Atom as a test setup for a web project, four spare HDDs= 1.4TB (2x PATA & 2x SATA)