I am looking for a web platform where people can submit feature requests for software. Visitors should be able to up-vote feature requests, in order for software developers to see the most popular requests. Is there anything like this? It would be awesome if it's also implemented in Node.js or Rails, but I am probably too picky here.
Two options are "Question2Answer" (open) and Google Moderator (hosted but free)
If it's open source and you want to host it in Launchpad, you can also use their Blueprints feature:
But I think you don't vote on this last one, and it may be too software-developer centered.
You could use FaultPress from WooThemes, and install any number of Voting plugins into WordPress so people could post feature requests and vote on them. Each feature could be associated with a Milestone that your developers could keep track of.