CodeDoor is a marketplace for freelance programmers where all contractors must be open source contributors, as defined as having at least one commit in the default branch of a public GitHub repository with 25 stars.
The profile page will prominently feature the contractor's open source commits, which acts as code samples for clients.
CodeDoor is open source: https://github.com/CodeDoor/codedoor
Richard Huang is a programmer who has created the Ruby gem Bullet, that automatically detects when your Rails app is making n+1 database calls, when one would do. He charges $90/hour: https://www.codedoor.com/programmers/109
Juan Manuel García created a web crawling framework, and a Python interface for the Facebook Graph API. He charges $33.75/hour: https://www.codedoor.com/programmers/7