MIT's Scratch is one of my favorite ways to introduce students to programming. I have used it in projects over the last 8+ years in after school classes, digital art classes and extensively in my computer science classes (part of the CSTEM program). I have used Scratch with 4th-6th graders, and they love it.
One of the biggest reason to use Scratch, is community. Students can share and learn from people around the world (millions of projects), and teachers can learn from the vast supportive teacher community that shares lesson plans and best practices. It comes with sound research and pedagogy from MIT.
Community! Community! Community! Encourages, sharing, remixing and learning.
Open, and really Free
Years of research from MIT
Used in schools, clubs and more for 5+ years
3 million+ projects to learn from
Free shared resources from teachers across the world
Vector graphics, sound editing - inspires original creations
Interfaces with Lego WeDo and other hardware
Multi-lingual support - remix projects made in a different language
Some of my favorite lessons
How do I? Help pages / More resources
My Scratch FAQ for use with students (answers to some common questions)
Some recommended resources to learn more on Scratch for teachers
Scratch Help page - check the starter projects, help videos.
ScratchEd :the best starting point for teachers, it has everything you need including stories, lessons, tips from teachers from all over the world.
NEW and updated! Scratch Curriculum guide from the Harvard Scratch Ed team. http://scratched.gse.harvard.edu/guide/
There are many resources from the Creative Computing Online Workshop Summer 2013:
Here is one of my favorites http://scratchables.blogspot.com/
Harvey Mudd's Scratch Course - MyCS uses Scratch https://www.edx.org/course/mycs-computer-science-beginners-harveymuddx-cs001x
Google CS First http://www.cs-first.com/