What resources would you recommend for K-5 Computer Systems Concept of the CSTA K-12 Standards?
I haven't seen a lot of resources in the US for that age group, though England seems to publish a great deal of high quality KS2 (Key Stage 2, ages 7-11) material.  What's particularly nice is the focus on Computer Science concepts and computational thinking over simple programming exercise, even at this young age.  Take a look at these BBC Resources from the "Bitesize" Series, which is a series of instructional videos appropriate to those ages).

There are also collected resources here, which includes such regulars as Scratch, but also points to some lesser-known resources, such as Code Club, which has projects appropriate for the age in various languages, and Kids Ruby, which teaches kids to program in the Ruby language with guided tutorials.