At the website The Griddle you can find a lot of different kind of Sudoku. The ones I love most are Curve-doku and Multi-doku, both Sudoku variants of Sum-Doku and Greater Than Sudoku.
About Curve-doku : This puzzles is Sudoku but with curved sections. This one has difference markers and greater-than symbols for clues.
About Multi-doku : This Sudoku variation contains two 6 by 6 cell grids which overlap in a 2 by 3 region. Each grid contains different clues, which include greater-than symbols and even digit markers.