Get Satisfaction and UserVoice are two very good (commercial) services involved into customer satisfaction, so they help companies to receive feedbacks about their products.
The second has also a free plan, even if with few features.
But there are various free alternatives, the following services have a free plan with more features:
But there are also full free services:
There are also some web scripts, but they require skills to use them:
Askbot another Python Django application, supports MySQL and PostgreSQL.
Question2Answer PHP + MySQL application.
LampCMS PHP application and run and use MongoDB.
OpenMind requires Ruby On Rails.
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