My favorite computer programming resource by far is Scratch. It allows students to be so creative and allows for wonderful problem solving. It also has amazing extensions that can be added for more complexity.
Grades 3-5: I use the Scratch Creative Computing Unit.
Grades 6+: I like to add robotics by using the Lego Mindstorm EV3 kits and Scratch extension to program the robots to complete these Robotics Task Cards. Thanks to Alexis Holt for allowing me to post!
Other favorites include:
Code.org - Probably the simplest place to start, with great "unplugged" activities, easy lessons that gently step students from one concept to another, and good navigation. Can start as young as kindergarten with this and advance through middle school.
Bricklayer - Grades 3 and up; combine math, coding, and art
Pencil Code - Grades 5 and up; create art, music, games, and stories using block coding