Stack Overflow - Community-based Q&A part of the Stack Exchange platform.

Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.

The Answer Bank - The Answer Bank is the UK’s leading question-and-answer site.

KanbanFlow - KanbanFlow is a Lean project management tool allowing real-time collaboration between team members. Supports the Pomodoro technique for time tracking.

Blurtit - Blurtit is a Q&A website. You can ask a question or answer on any question.

Meistertask - MeisterTask is an intuitive task management and collaboration tool.