Minopoli has created a significant sudoku variant Sudoku++ see:
http://joujoujou.over-blog.com/article-25839168.htmlI really enjoyed the puzzle so I thought I'd have a go at creating one. Quite difficult to create the solution - creating the puzzle was not then too difficult.
Rules:
The yellow squares are a sudoku that obeys the row, column rules for all eighteen rows and all eighteen columns also the red boxes contain one to nine in the yellow cells.
The white squares are clues and the value is the sum of the orthogonally adjacent four (or less) yellow cells.
For killer addicts remember that the sume can represent repeated numbers if it crosses a box boundary.
Not too difficult once you get the feel for it.