I have a stack, 12, of rack-mount servers that range in specs.
Lowest: dual 500mhz Xeon 2gb RAM SCSI 3x18gb
Mid: dual 1ghz Xeon 4gb RAM SCSI 4x18gb
Highest: quad 900hz Xeon 4gb RAM SCSI Raid 4x36gb

All are about 8-10 years old and are in good working order and run a Dell PERC/II or PERC/III RAID card. Each has dual onboard 10/100 Intel Server NICs and some have additional Intel server PCI NICs. I also have 30 spare 18gb SCSI Drives and 4 spare 36gb drives as well as almost everything else as a spare. These were the best of the lot that I rebuilt and stress tested. They've all been replaced by newer hardware for production use. I have the room and the additional power consumption, heat, and noise aren't issues. I would like to do something useful with them rather than let them rot or throw them away. I tried to donate them to some local schools and non-profits but no one would take them, even offering to install Linux, setup SAMBA, OOo, Firefox, etc. and showing them how to use it.

I already have production servers for pfSense, Apache, terminal services, active directory, SAMBA, mySQL, MSSQL, asterisk, email. Some even have a failover.

So, what could I use these for? Beowulf cluster (not sure what I would do with it)?

Maybe run a few distributed computing projects like SETI, Einstein or folding@home.

I've run SETI on my allways-on machines since I first heard of it in 2002. I also have electric sheep on my laptop.
I was looking for something more useful for me.