Below is a website that was shared with me recently by a Math Goddess!
It's an excellent site loaded with links different types of elementary math lessons, applets and projects.
There's a great page on manipulatives and the role virtual manipulatives play in the mathematics classroom:
In addition, here's a link to a great article by Douglas H. Clements:
Concrete Manipulatives', Concrete Ideas. http://gse.buffalo.edu/org/buildingblocks/Newsletters/Concrete_Yelland.htm
There are parts in this article, particularly regarding the role of the teacher in planning and implementing programs that embed virtual manipulatives, that you might find interesting.
I would love to hear your thoughts about the contents of these articles and your work with virtual manipulatives.