ABCya: This sites helps teach students about using technology... activities organized by grade.
Internet4Classrooms - Kindergarten: This is an extensive and well maintained resource repository (Kindergarten)
Internet4Classrooms - Grade Level Skill Builders: Another way to look through the vast resources of i4c (K-8)
BBC Schools - Primary Grades: This site has a wide variety of links in all subject areas for students (ages 4-11)
K-3 Web Site...ings of the Day: This is a blog that shares wonderful resources and tools and information about them (Teacher)

Reading:
Robert Munsch - Storytime: A large list of readings you can listen to right on the computer (K-3)
Starfall: A favorite with K-2 students and teachers.  This interactive site helps children learn to read (K-2)
Roy the Zebra: similar to Starfall for emergent readers..literacy, games and worksheets...
StoryPlace - Elementary Library: Encourages students to listen to interactive children's stories, play with on-line activities and print out take-home activities
FableVision: This site has so many wonderful reading related resources that teachers can share with their students! (K-4)
SuperThinkers
Enlighten Me
3rd Grade Core Words

Math:
Everyday Mathematics
Arcademic Skill Builders

Tools:
4Teachers: This site is a real time-saver; providing a whole toolbox of educationally purposed tools for teachers (rubric builder, quiz maker, and more). Also, many of the tools can be used by students to structure assignments.