For about fifteen years I ran a "Puzzle of the Month" with prizes for correct entries. I have now collated the puzzles by category. You are welcome to download them and either try them yourself, or use them as challenges for students. The files are "hybrid PDFs" which can be viewed with any PDF viewer, or edited if opened with LibreOffice or OpenOffice. There are no answers - deliberately! It's much more satisfying to work out a puzzle for yourself.
In some years the puzzles were subdivided by age group:
Junior Puzzles aimed at (English) school years 7, 8, and 9.
Intermediate Puzzles aimed at (English) school years 10 and 11.
Senior Puzzles aimed at (English) school years 12 and 13.
In some years the puzzles were subdivided by topic:
Number Puzzles - Cryptarithms, Factor puzzles and many other types
Shapes Puzzles - polyominoes were always particularly popular.
Algebra Puzzles - using equations to solve all sorts of puzzles.
Miscellaneous Puzzles - mostly involving logic rather than calculation.
There is also a page with a selection of Christmas Puzzles to choose from.
There is a small amount of duplication - for example a "Junior Puzzle" might also be a "Number Puzzle", but that doesn't happen very often.
Some puzzles I made up myself, others were culled from all sorts of sources, most of which I can't remember. So if I have infringed anybody's rights by posting a particular puzzle online, I apologise. If necessary, please contact me and I will remove it.