If you are searching for a simple and easy to use program, Popplet is probably your go-to XMind free alternative. While there are only a handful of features, you can still create comprehensive mindmaps. Using this program, you can invite collaborators for meeting and brainstorming provided they have a Popplet account.
SciAmp
Popplet
Padlet
Quizalize
Kids A-Z
Acadly
Seesaw
BookWidgets
Pingo
