Question: I need a free wordpress plugin that will allow the users to upvote or down vote every post. Something similar to stackoverflow.com

If not give me suggestions of how to create one. I am a java programmer and have never coded anything in PHP.

Answer: 
[GD Star Rating](http://wordpress.org/extend/plugins/gd-star-rating/) ( plugin is no longer in development ) was pretty comprehensive.
Can't answer completely now, but i could achieve a vote on/off functionality by implementing a "custom comment type". 
It behaves very much like a comment, except it doesn't get listed/counted as a normal comment and can be retrieved independently from the comments table.

If you are interested I can post some code (quite some hacking into it, so i'll post it if there's some interest).