The most common open license is the Creative Commons license.
What is Creative Commons? "Creative Commons helps you legally share your knowledge and creativity to build a more equitable, accessible, and innovative world. We unlock the full potential of the internet to drive a new era of development, growth and productivity."
"With a network of staff, board, and affiliates around the world, Creative Commons provides free, easy-to-use copyright licenses to make a simple and standardized way to give the public permission to share and use your creative work–on conditions of your choice.”