The Answer Bank - The Answer Bank is the UK’s leading question-and-answer site.
Cocorico - Cocorico is a state-of-the art Open Source solution to create an Airbnb or Blablacar for anything.
Blurtit - Blurtit is a Q&A website. You can ask a question or answer on any question.
Sharetribe - Build your online marketplace business. You don't need a developer.
Ask Me Help Desk - Ask Me Help Desk is a question-and-answer site.
JaJuMa-Market - All-In-One Multi-Vendor / Online Marketplace Solution. Various Business Models supported.