performing online surveys are important instruments for gathering your clients opinion, demographical data and in general learn more about your audience...

I found the following open source solutions for later evaluation:

PHPSurveyor

• Unlimited number of surveys at the same time
• Creation of a printable survey version
• Unlimited number of question groups in a survey
• 20 different question types with more to come
• Ability to set conditions for questions depending on earlier answers (branching the survey)
• Re-usable editable answer sets
• Ready-made importable questions
• Assessment surveys
• Unlimited number of participants to a survey
• Anonymous or open surveys as well as closed surveys
• Optional public registration for surveys
• Sending of invitations, reminders and tokens by email
• Option for participants to buffer answers to continue survey at a later time
• Cookie or session based surveys
• Back-office data entry possibility
• Survey start & end-dates for automation
• import and export functions to text, CSV and MS Excel format
• Basic statistical and graphical analysis with export facility
• more than 15 different languages

phpESP

ModSurvey xml based survey configuration

Open Survey Pilot another nice solution for survey creation, altough these have less diffent question types and a bit more instable look/feel than PhpSurveyor... the german main page is here