This site is a strong tool for instructing and practicing math. It includes an enormous list of lessons and topics and allows unlimited practice. Students also receive immediate feedback on their work. A downside is that there is not any tracking of progress.
This is one of my favorites that I use quite often in my classroom. It includes numerous virtual manipulatives that can be used for whole class lessons and that students can also use individually.
This is another online resource of virtual manipulatives. You need not have the Houghton Mifflin Math program in order to use them. I prefer this site over NLVM for working with coins.
This site has quite a few interactive games. Students can play versus the computer or against online users. I have not tried it, but it says that these games can also be played on the Nintendo Wii when you access the site through the Wii browser.
I use this site often to create word searches, math work sheets, and some graphic organizers. It is a free site now, but says that it is still in beta. I've been using it a little over a year now and they are still continuing to grow.
Pennsylvania Department of Education's Standard Aligned Systems site includes numerous resources for all subjects. Resources include lesson/unit plans, videos, web-based content, educational resources, instructional content, PA educator created content, and assessments.
Illuminations includes a large number of math lessons, games, and interactive activities. It is presented by the National Council of Teachers of Mathematics.
I do not have a lot of experience with Math Fact Cafe, but it appears to offer numerous possibilities for creating worksheets and flashcards. It also features pre-made pages for math facts.
Aplusmath.com features a number of Java games including a math bingo game, hidden pictures, concentration, and a planet blaster game. I have used this site for full class lessons and individual practice.
Study Island is a pay service site that features deep assessment and game tools. I have used this at length this year and would recommend it. I am able to assign tasks to my class or individuals. I am also able to create new classes for my RTII groups. Students can do the lesson for each topic, take a test, and then play games for practice. All of the answers are tracked for the teacher to view. If a student has had trouble earlier you can also delete previous attempts.
Promethean Planet is a strong community of interactive whiteboard users who create and share flipcharts that they use during their lessons. You do not need to have an interactive whiteboard to use some of these activities.
Timez Attack is a role playing style game that requires students to answer different multiplication problems in order to advance through gameplay. This is the most "video-game-like" educational game I have seen yet.
This site is another collection of online games and practice. Like others, it does not allow for tracking scores over time. My favorite part of this site is the selection of interactive 100 charts. I have used these often in class.
Math playground is another solid practice/game based site. I have used it minimally to this point, but intend to give it a try with my class. It appears to be well created and organized, but like AAAMath does not allow you to track student progress over time.
Dr. Mike's Math Games for Kids includes some online games and also many ideas for games that you can create and play in class or at home.
ABCteach includes many different math worksheets that you can print and use in the classroom. You don't seem to be able to edit them at all and many are only available to paying members of the site.
FunBrain has a large collection of well-made educational games. My students love using this site. However, teachers need to monitor what games students are playing because some are not strictly educational.
PBS Kids includes a large number of math based games. My favorite review game to use whole class is Lucky Star. We work together and do one question at a time on white boards and I can monitor student answers before we submit one.
ABCya! is another collection of well-made online games. They all appear very kid friendly. To me the only downfall is that they don't seem to be very well organized.
Funschool, like ABCya! and others is a solid collection of well-made online educational games. These games are fairly well organized and kid friendly.