I run Boinc on 7 Slackware systems: Slack 11.0 up through 13.37. World Community Grid.
The 11.0 boxes need an older version of the Boinc client, but I usually only run them as the temp goes down, and the PC BTU's help keep my home warm.
The oldest system is a dual Pentium Pro - 200 MHz w/512K cache. Not very productive, but generates some serious warmth!
This SAME system participated in the DES challenge way back in in the late 1990's!
Other systems include a couple of IBM ThinkPad A21p's. P3 1GHz, an IBM Netvista X40 with a P3 1GHz, and an IBM ThinkPad A31p with a P4M 2.6GHz. The other's are AMD dual cores 2.4 GHz.
I shut most of these systems down in April. I anticipate that I'll fire them up in late October or November. My electric bills shoot up about $100 USD/mo. to run all of this equipment.
Cheaper than buying heating oil, and benefits humanity at the same time. That's a big plus to me.
My other four systems at home run Slack64 13.0, with AMD quad/hex cores for Folding@Home (as larryb_home). Their main purpose is to run MythTV 0.20, and hold a total of 7TB of storage of TV programs that I record on the side. I shut all but the main server down in the summer, after the TV season is over. These also help out in the heating of my home during the winter.