Math is Fun is an outstanding website that might supply a large part of your math curriculum, and it is free. While it has some unobtrusive advertisements, even these can be turned off. Be aware that many of the games require Flash, so it won't work on some devices.
The home page has buttons for another index page as well as for specific topics: data, numbers, puzzles, money, games, dictionary, worksheets, measurement, activities, geometry algebra, calculus, and physics. If you just want to use the site for supplemental teaching, games, or activities, you can easily get to these from the home page.
If you want to use the site content more methodically, click on the index button. You can locate lesson material in a few different ways. The first display shows groups of grades levels (PreK-2, 3-6, 7-9, and 10 upwards) with six tabs under each grade range: numbers, algebra, geometry, data, measurement, and money. Or you can click on Common Core Standards and find grade level or topic tabs for kindergarten through high school. (High school level tabs list algebra, functions, geometry, number and quantity, and statistics and probability.) Clicking on one of these tabs brings up a page listing Common Core standards, each with multiple links beneath it that take you to instruction or activities that are tied to that standard.
Instead of clicking on Common Core Standards, you might prefer to click on “Curriculum.” The site explains that the curriculum was originally based on NYSED Curriculum (New York State Education Department), but it has been altered to make it more international. On the curriculum page, courses are listed for PreKinder through Year 8, plus high school courses for algebra, algebra 2, geometry, statistics, and calculus. For each grade level or course, topics are listed under headings such as these for Year 3: counting, addition, subtraction, multiplication, division, numbers, fractions, measurement, time, geometry (plane), geometry (solid), pre-algebra, data, estimation, and money. For money lessons, students can click on a world map to identify the country’s currency that will be used. (As a side note: the currency activities would be very useful for upcoming travels abroad!)
Topics generally begin with an instructional lesson, but not always. This fairly thorough instruction is generally followed with “Your Turn,” a set of ten practice problems. However, more complex topics, especially at high school level, have links at the end to “Here’s More” pages. These are additional pages of instruction and/or activities, and a few pages have practice problems. The amount of material here is simply amazing.
The Worksheets tab offers options for creating online or printable worksheets on narrowly defined topics. Some worksheets must be printed, and some can either be completed online or printed. There is usually an option to print out the companion answer key for a worksheet. Otherwise, you might bookmark a particular worksheet and return to it to access its answer key.
Practice isn’t limited to traditional problem solving. Games and interactive activities are used frequently. Young children can begin with connect the dots, counting bugs, a “Connect Four”-style game, or other age-appropriate games. Older children play patterns games, multiplication games, a form of chess, Sudoku, battleship, and lots of other math and strategy games. Logic puzzles are also available under another large category of their own.
A “Data” button takes older students to pages for learning about handling data, especially charts and graphs. There are also online graphing and display tools for students to use.
While Math is Fun has great content, it is not intended to provide comprehensive courses. It doesn’t direct students to particular lessons but leaves them free to choose whatever they wish. There is no method of cumulative review, a necessary element of a complete course. Also, it does not track student progress. Nevertheless, the amount of material here is very close to being comparable to complete course material, particularly for the elementary grades. There are other math sites with free activities, but I have not seen a free site as comprehensive as this, nor have I seen many with interfaces as clean and easy to use.
Check it out to see if it might be useful for you!