There are many recommendations for Nrich games on the pages of this site. Use the search and filter by Type, Age and Challenge level.
Note this selection of Games chosen by Nrich themselves, Secondary Interactive Resources which have been chosen for use in the computer room or on the whiteboard. The games are presented in order of difficulty; we have Countdown for example, but with Complex Numbers, Vectors and Matrices.
For very attractively presented Puzzles and Games – try Mathigon.
Jonathan Halls MathsBot.com includes a collection of Puzzles Games and Activities.
On Sarah Carter’s Math=Love site, you will find a wonderful collection of puzzles. The collection is helpfully indexed into several categories including Puzzles Related to Math Content and Number Related Challenges. For the Shared Factors puzzle illustrated here, John Golden created this GeoGebra version, Quadratic Puzzle.
Radio 4 included a Puzzle for Today feature at one time including many excellent puzzles, you can still see many of the puzzles.