Do you mean where members rate others' posts? Why not look at something like Slashcode [slashcode.com] or Scoop [scoop.kuro5hin.org]. Neither are particularly easy to set up, but both are free/open source.
I haven't seen anything specifically for voting on content, but you might be able to use either a "poll" or "thread rating" feature if each content item was in its own thread. These functions are either built-in or available as hacks on various forum software packages.