Jenkins Amazon EC2 Plugin
wiki.jenkins-ci.org/display/JENKINS/Amazon%2BEC2%2BPlugin
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
Allow Jenkins to start slaves on EC2 or Ubuntu Enterprise Cloud (Eucalyptus) on demand, and kill them as they get unused.With this plugin, if Jenkins notices that your build cluster is overloaded, it'll start instances using the EC2 API and automatically connect them as Jenkins slaves. When the load goes down, excessive EC2 instances will be terminated. This set up allows you to maintain a small in-house cluster, then spill the spiky build/test loads into EC2 or another EC2 compatible cloud.