MIT's Scratch is one of my favorite ways to introduce students to programming. I have used it in projects over the last 5 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 (3 million projects and counting are online), and teachers can learn from the vast supportive teacher community that shares lesson plans and best practices.
Some of my own lessons / projects in Scratch that I have used in my classes :
( I am also working on a set of 'programming through art' series with lesson plans etc for Scratch 2.0. Let me know by email if you are interested and I will send you a link.)
Some recommended resources to learn about Scratch/teach Scratch
Scratch Help page - check the starter projects, videos.
ScratchEd - the best starting point for teachers, it has everything you need including MIT"s own Scratch curriculum, stories from teachers using scratch, and some great teaching tips.
Computer Science concepts in Scratch : A Free textbook on Scratch, well written, detailed and comprehensive approach to teaching computer science. Download PDF version available. (Hebrew version available as well)
Learn Scratch : great resource of online video tutorials (In English and Spanish)