You probably know that all Drupal core patches and commits get tested using the simpletest testing system, but you may not know how. Essentially, every time a patch is posted and placed in the "Needs Review" status on Drupal.org, and every time a commit is made, the information is sent off to qa.drupal.org, which then farms it out to testing working machines we call testbots.
Well, the testbots are quite compute-intensive, and although we tried for years we never really got enough decent, manageable machines from community contributions. Partly this is because the machines are really compute-intensive, and partly it's because they can be pretty tweaky and not everybody wants to learn how to manage them and deal with their occasional fits. Many community members had donated time on their machines, but in recent months we'd been using Amazon EC2 instances for the bulk of our testbots, and the bill to the Drupal Association was more than we'd like to pay, more than $500 many months.