WebMath
This is a great resource for any level of math problem, from counting to calculus.  Just choose the math category, put in the numbers from the problem you are attempting, and WebMath will provide a step-by-step guide of how to solve it! 
Skills: covers all levels of math through calculus

Math.com    
This is another wide-ranging site.  Select a topic, from "everyday" math through calculus, and use the tabs across the top to see tutorials and work practice problems with hints.  The "calculator and tools" tab offers many different converters (liquid measures, distance, weight, temperature), application-specific calculators (interest rates, integrals, probability), and graphers. 
Skills: covers all levels of math through calculus

Visual Math Learning                 
This site provides lessons, practice exercises, games and puzzles on topics from basic addition through algebra.  All of these lessons are read aloud to the student, and animated numbers move to illustrate concepts like borrowing, regrouping, and balancing equations.
Skills: addition, subtraction, fractions, decimals, integers, equations, variables, and functions.

 Coolmath              
Coolmath has easy-to-follow lessons and examples for pre-algebra and algebra topics, as well as math games and practice activities for addition, subtraction, multiplication, division, decimals and fractions.
Skills: addition, subtraction, multiplication, division, decimals, fractions, exponents, polynomials, radicals, systems of equations, quadratics, absolute value, lines (slope, graphing), and inequalities

AlgebraHelp 
There are interactive worksheets for many algebra problems.  Work the problems at the computer, enter your answer, and check your answers.  If you need extra help, links are provided to step by step examples.
Skills: porportions, exponents, polynomials, quadratic formula, graphing

Scott Foresman mathematics: used in grades K-6
Each grade has a "Take It to the Net" site with virtual manipulatives in the "eTools" section, multimedia illustrations of working problems in the "examples" section, and practice quizzes with instant feedback in the "Test Prep" section.   Note: Some teachers will create specific assigned activities for their students and distribute passwords for access.  Otherwise, click on your grade here:
Kindergarten       First        Second        Third          Fourth            Fifth         Sixth

Mr. Myers' Math Mania Facts Practice 
Scroll down to the "Online" practice section and choose which operation to practice.  You can even set a timer at the top of the screen.

 Spacey Math               
Practice facts from any of the four operations (addition, subtraction, multiplication, or division) at this site.
Skills: addition, subtraction, multiplication or division facts

Leon's Math Dojo  
This site offers a wide variety of math activitities in the areas of counting, addition, subtraction, multiplication, and division.  This link to the main page allows you to select an activity by skill within these main areas.  Note:  Click on "maybe later" when asked to register to avoid having to create a password.
Skills: counting, skip counting, adding (facts, through 2 digit with regrouping), subtraction (facts through 2 digit with borrowing), multiplication (facts, two digit by one digit, by 10's), division (with and without remainders through 12). 

Math Stories 
Practice word problems at this site.  The problems can be read aloud, and visual cues for finding the important information in the problem are offered.  Note:  Click on "maybe later" when asked to register to avoid having to create a password.
Skills: Identifying needed information and setting up equations from word problems - choose either addition and subtraction, or multiplication and division.

Line Jumper    
Solve math problems using an onscreen number line.  Levels of difficulty go from adding and subtracting to 10 through using positive and negative numbers through 16.
Skills:  adding and subtracting facts to ten, adding and subtracting integers

Place Value Workshop 
Use these virtual blocks representing thousands, hundreds, tens, and ones to add and subtract with regrouping. Note:  Click on "maybe later" when asked to register to avoid having to create a password.
Skills: place value to thousands, regrouping and borrowing

Math Movies 
Solve math problems to help the hero escape traps in these adventures.  Choose the type of problems to practice.  Virtual manipulatives can be turned on to help solve the problems.  Note:  click "maybe later" when asked to register to
avoid having to create a password.  
Skills: counting, skip counting, adding or subtracting (facts through 2 digit with regrouping), multiplication (facts through 2 digit), division (with or without remainders)

Math Arcade from Funbrain 
This is a linked set of 25 games to practice computational skills.  Choose one of 8 levels, then start the first game.  You must "win" each game to go on to the next, while the game board on the left keeps track of your progress.  Each player is given a password (without giving personal information) which allows you to resume play where you left off on the next visit to the site.
Skills: computation with all four operations (difficulty depends on the level chosen )

Decimal Beat the Clock 
Type the decimal represented by a shaded grid.  Several levels of difficulty and speed.
Skill: identifying decimals from tenths to thousandths

Mission Magnetite 
Match fractions, percents, and shaded figures with this game. 
Skill: converting fractions to percents, representing percents and fractions with shaded figures