These are a few sites with interactive activities that may be valuable for primary grade students. Use these for 15 minute center work with the computers in your classroom. Many of these activities would be wonderful as a whole class activity on an interactive white board!

Language Arts

Molly Mix-up: Work with a partner. One person types a sentence, one work in each box and clicks "Begin". The partner then tries to figure out what the sentence said by putting the words in the correct order. This could be used to practice the skill of writing complete coherent sentences.

Houghton Mifflin English
Kid's Place

Reading

Starfall
The StorePlace
CBeebies - Tweenies
Try Story Time, Messy Tiime, and Game Time

Math

How Many Were Taken Away: Practice basic subtraction with beginning algebraic formulas. The student has to complete the math sentence by figuring out how many aliens were taken away
Rainforest Math

Science

Magnazone: Students find objects that are magnetic.
Simple Circuits: Students use objects to create simple circuits.
BBC Schools Science Clips

All Subjects

Digger and the Gang

Here's a great presentation that gives good information: