Since you're asking this question, I'll assume that you have the freedom to do whatever you want with them. We'll assume they're your servers, personally. In that case, keeping them at the ready in case your cloud solution turns into a hurricane is a great idea that was mentioned previously.
Otherwise, a couple of things come to mind:
1) Start a web hosting company, using Linux and cPanel
2) Start a Private VPN service
3) Beowulf Cluster! (this is slashdot, after all...) or the modern version: OpenStack