What are some of the best places for free stock photos

What are some of the best places for free stock photos

Adding images or photos to content, blogs or sites is very important. It increases its effectiveness, improves its presentation and readability. However many find problems when it comes to find sites where they can download royalty free stock images.
So I would like to ask which ones do you use? I did some research and it seems that some of the best ones include
Picjumbo, Stockpic, LibreShot and Unsplash. Did you try any of these? Can you recommend any others?