These math websites cover a range of online activities as well as printable materials:

http://jmathpage.com/index.html  - links to interactive math sites and games

http://www.scholastic.com/kids/homework/flashcards.htm - make your own flashcards for math (or any subject).  Print them out for studying on the go!

http://www.infoplease.com/math/flashcards.html - practice math flashcards online.

http://www.numbernut.com/index.html  - lots of drill and practice online

http://www.mathlab.com - online lessons, games, printable games and activities for teaching all areas of math.

http://www.math.com - lots of activities to support math homework and practice.  Choose information based on an overview or more in depth look.

http://www.incompetech.com/graphpaper/ - free downloadable graph paper, along with many other types of paper (handwriting, dots, notetaking paper, etc.)

http://illuminations.nctm.org/ActivitySearch.aspx?grade=all - online activities searchable by grade or type.

http://www.aaamath.com/ - a comprehensive website of interactive lessons and practice for Kindergarten through 8th grade.

http://www.ptolemy.co.uk/software/furbles/?flash/exploreDemo.swf  - attribute shapes that you can organize into pie and bar charts. This flash demo version allows you to interactively explore data with engaging little creatures