What is is it?
Engineering effort applied to freely publish useful engineering data and designs such that others may freely reuse the information.
A wood stove design others can build, modify and deploy as they please.
Manufacturing information for a water desalination device that others may modify, build, republish, or deploy for personal drinking water or to Terraform the Sahara or Mars.
Free as Beer or as in Lack of Restrictions on Use of the Data?
How do free engineers get paid?
Any way they can devise without restricting the data.
How is this possible?
There are a lot of people around with engineering related experience and education. Many free open source tools are becoming available via the internet at Sourceforge and elsewhere.
Who else uses open source or free development of the knowledge commons to reduce future investment requirements in using technology effectively?
Why would anyone contribute time and expertise for free?
Some people want to see space settlements in their lifetime. Others have other personal agendas like sound ecological management or sustainable human civilizations on Terra. Tell us your goals or reasons!
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Examples of Free Engineering Projects 
- algOS - Oil from Algae Open Source
- Open source car projects: