Non-Commercial (Open Source)
Django Voice (Python; BSD Simplified)
All Our Ideas (Ruby; Modified BSD)
IdeaTorrent (defunct)
Openmind (Ruby; GPLv3)
Non-Commercial (Closed Source)
Google Moderator (Widget and API)
Commercial (Open Source)
???
Commercial (Purchase Source)
VoxPopuli (PHP + MySQL; License Agreement)
UseResponse (PHP + Zend; License Agreement)
Commercial (Closed Source)
IdeaInformer
UserVoice
UserReport
UserEcho
BetaEasy
CrowdSound (defunct)
GetSatisfaction
You could also use the voting system of one of the many StackExchange Clones, my favorite being Question2Answer. For a full list of SO Clones, check out this SO question.