A new craze has hit the UK: Kakuro. It is a mathematical crossword, similar to Sudoku - except you have to be able to add as well! They are a form of linear programming problem - though they are much more fun when solved using a pen and pencil! The Guardian was the first newspaper to introduce it to Britain - though it remains to be seen if it will replace Sudoku. I got hooked on Sudoku over the summer while we were away camping and I'll be trying Kakuro over the Christmas holiday. Some games are available here, here ,here and here. There's even a tutorial here.
An on-line version is available here; and a list of valid digit combinations are here.