Redmine is powerful bugtracker containing a case tracker, document management, wiki, versioning GUI, RSS... and so can be used as a platform for developer communication.
Those plugins below allow you to manage contractors easily. You can set up a rate for each and when they track their working time against tasks, you can later generate a report with the Timesheet plugin and get a detailed overview of tasks, total time taken and total cost.
Budget
Invoice plugin
Rate Plugin
Timesheet Plugin
The original document is available at http://www.heraprocess.doleans.net/tiki-index.php?page=using_redmine