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.
Asana - Asana project management is an effort to re-imagine how we work together, through modern productivity software. Fast and versatile, Asana helps individuals and groups get more done.
Blurtit - Blurtit is a Q&A website. You can ask a question or answer on any question.
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.