Server Fault - Server Fault is a Q&A site for system administrators and IT professionals that's free.
UserGuiding - Create in-app experiences with the most straightforward product adoption platform — quick implementation, lasting user engagement.
Reddit - Reddit gives you the best of the internet in one place. Get a constantly updating feed of breaking news, fun stories, pics, memes, and videos just for you.
Usetiful - Fight user churn with great user onboarding. Interactive product tours and smart tips significantly improve your user retention.
MetaFilter - Community weblog
Userlane - Digital adoption platform with interactive guidance for software applications allowing anyone to understand and master any software from the get-go.