Fact Monster-
http://www.factmonster.com/games.html
Select "math games" or "math flashcards."  I like the Fraction Cafe and Cowboy Math for my 6th and 7th graders.  I would choose those two for my younger students because in order to play the game you have to do math.  The game isn't a reward for doing the math, the game and math are interwoven.  I would use the Baseball Geometry with my 8th graders.  I don't think I'd do the Baseball Geometry with my younger students because they'd be more concerned with playing the game and less concerned with the math part of it.  The 8th graders I can trust to do the math before playing the game.

Math-Play
http://www.math-play.com/

The middle school games are created to be like popular television games (jeopardy, millionaire, etc.) or sports (soccer, football, etc.), and the games are organized by grade level.  In the middle school games, the 6th grade games are much better than the 7th or 8th grade games.

SoftSchools.com
http://www.softschools.com/math/games/
I would use these games for some of my lower students (since I have 5th graders that are 2 grade levels behind).

Sheppard Software
http://www.sheppardsoftware.com/math.htm
The games on this website are organized in a way that students can easily choose a skill they want to work on.  I like the website because it's organized by specific math knowledge and skills, and not so much by grade-level.


My 5th graders are currently working on long division.  Double digit long division is a begining skill for 5th grade and it's difficult for them.  Some of my students understand division and can follow the steps perfectly and get the right answer every time.  For those students who still aren't getting it, there are some games on the websites I found that should help them with some fun practice.

My 6th and 7th graders are working with fractions, decimals, and percents, and there are a ton of interactive games that deal with fractions!

My 8th graders are headed towards geometry, and there are a lot of resources on the above websites that are geared toward shapes, angles, area, perimeter, etc.