I love online manipulatives! They don't cost anything, you don't have to store boxes of little pieces and parts, and they don't roll under desks. This collection of Math iTools by Houghton Mifflin is an excellent resource for any K-6 classroom.
There are 10 main sections starting with basic counters and ending with probability activities.
Within each section, you will find easy to use activities and fun graphics to engage your learners while reinforcing important math concepts.
There are also built in activities within each section to further practice each concept.
I know you will want to add the Math iTools resource to your lessons for small group and individual practice.
For more interactive, online math resources, visit my Interactive Sites for Education.