Edit: This initiative wasn't really successful, but since this was written the testbots migrated onto the OSUOSL Supercell infrastructure. The center for information about the testbots is the Testbot project and issue queue.
Do you have some spare computing power, or want to provide some to the Drupal testing infrastructure?
You probably know that every commit and every patch submitted to Drupal 7 Core gets a full test (more than 20,000 assertions) taking 25 minutes on non-trivial computer hardware. So we can generally use more computing power. For the code sprint at Drupalcon Copenhagen, we had 13 machines testing patches so that everybody could get the fastest possible turnaround.
Anyway, it's not hard to do it. You can either install a machine with Debian 5 and go through a simple setup, or just use the free Virtualbox program with a pre-set configuration. That way you don't have to blow away an existing machine's configuration, and you can still return it to its old life after it's served as a PIFR client for awhile. (PIFR is Project Issue File Review).
There is a new writeup on qa.drupal.org explaining how you too can provide a testbot. Your testbot has to be able to reach the internet (and qa.drupal.org) but it does not have to be reachable from the internet, and it does not have to have a fast internet connection. It just does its work and then returns the results.
The only thing we ask is that you take care of your testbot. Don't turn it off without first disabling it on qa.drupal.org. Don't forget about it and have it all broken and everything up there. It makes things complicated and it makes a mess on http://qa.drupal.org/pifr/status :-)