http://www.otl.pps.k12.or.us/teachers.pps.k12.or.us/math/elementarymath/software_supp.html

This link goes to a list (by grade level and strand) of online math activities that support the Investigations curriculum.
They have a wide range, and it is K-5.

http://nlvm.usu.edu/en/nav/vlibrary.html

This site is often listed by schools as a good resource for math manipulatives. These do require "Flash" software on your computer, but you should have that already. If not, it is easy to download.

http://www.everett.k12.wa.us/math/Elem.%20Resource%20Links

This includes a wide variety of links, including NLVM, Investigations, and also assessments and rubrics.

You may also want to look at the links on the Everett math site by grade level. Use the following links for that.

Kindergarten	First Grade	Second Grade
Third Grade	Fourth Grade	Fifth Grade

Everett also has an online parent resources section. You may find more of use there-

http://www.everett.k12.wa.us/math/Online%20Resources

You will also be able to locate many useful math activities through Google or other search engines. Simply type in something like this: