Illuminations is a website by the National Council of Teachers of Mathematics. They have many great online activities that address many math topics. Here are a few that I found to be pretty fun.
FactorizeWith this activity, you can visually explore the concept of factors by creating rectangular arrays with an area equal to the product of the factors.
The Product GameThe Product Game is a fun, interactive game that exercises your skill with factors and multiples.
Primary Krypto GameThe rules of Krypto are simple: Combine five number cards using the four arithmetic operations (+, –, ×, ÷) to arrive at a "target" number. This online version of Primary Krypto uses the numbers 1–10 only.
ConcentrationPlay by yourself or against a friend to form pairs. Match numbers, shapes, fractions, or multiplication facts to equivalent representations. The game can be used to practice facts by using the clear pane mode, or for an added challenge, play the game with the windows closed.
Equivalent FractionsCreate equivalent fractions by dividing and shading squares or circles, and match each fractions to its location on the number line. This is a
great activity to practice seeing equivalent fractions.