Super Math World isn’t a totally free site but it does have a LOT of free content that kids can access.
Free topics include: adding, measures, number patterns, percentages, place value, area, estimating, fractions, negative numbers, set theory, venn diagrams, and series. The kids will enjoy the arcade-like practice area.
How to integrate Super Math World into the classroom: Super Math World makes a great computer center activity during math. The games are quick-enough that students can filter through classroom computers for their turn over a few periods. The games would also make fun whole-class interactive whiteboard games. These are intended to be one player, so you can split your students into teams and have them take turns at the board.