freedomsponsors-trac-plugin
This is the FreedomSponsors "SponsorThis" plugin for Trac.
When installed, it will add a "Sponsor this" link in context navigation bar on your Trac's ticket view. The link leads to a FreedomSponsors page where the user can place a bounty for that ticket.
Similar functionality is also available for JIRA, Bugzilla and Github.