title: Free alternative to Atlassian Greenhopper/PivotalTracker?
body: <p>I'm working with my brother on a website idea we have, and we'd like to use a tool to plan sprints and assign estimates to individual tickets.</p>
  
  <p>Atlassian's JIRA+Greenhopper looks fantastic but costs $20/month and at this stage we're just validating our idea and would rather not spend money on a tool if we can avoid it.</p>
  
  <p>Are there any free alternatives to these tools?</p>