Free online survey and diary software for conducting experience sampling studies Are there any open source online survey software that enable researchers to use experience-sampling methods, e.g. online diaries (where people answer similar survey questions repeatedly and are notified and reminded)? Free, but not open source, would also be interesting.

Please only post solutions, that you have actually used for experience-sampling research, not ones that "might be easily adapted".
The best review of experience sampling tools I've found is here.

Specifically, to answer you question, check out "MyExperience". To quote the website:


  MyExperience is a BSD-licensed open source mobile data collection tool
  developed for Windows Mobile devices (including PDAs and mobile
  phones) using .NET CF 2 and Microsoft SQL Compact Edition.
  
  MyExperience is available for free on SourceForge in beta release.
  Please see our wiki for documentation and our blog for updates.
  MyExperience combines sensing and self-report to collect both
  quantitative and qualitative data on human behaviors, attitudes and
  activities in the field. Using a mobile phone's wireless connectivity
  to the internet, researchers have the ability to access MyExperience
  data as it's collected allowing for ongoing analysis of study data and
  early detection of subject compliance or technology issues.
  
  MyExperience is based on a three tier architecture of sensors,
  triggers and actions; triggers use sensor event data to conditionally
  launch actions. One novel aspect of MyExperience is that its behavior
  and user interface are specified via XML and a lightweight scripting
  language similar to the HTML/JavaScript paradigm on the web.