Number puzzles are worthwhile at every grade level, but they are especially useful in middle school, where they simultaneously provide a chance to review topics in arithmetic, practice problem solving, and get acquainted with the power of algebra.
Here are links to the number puzzles you can find on this website.
Some are lessons or sidebars in Algebra: Themes, Tools, Concepts. (The whole book is available as a free download for your non-commercial use.) In some cases, you will need to scroll down to get to the puzzles.