The sudoku of the day is greater-than sudoku, where you get no clues; instead, every box is marked greater-than or less-than its neighbors. If you need more than the ten at the link above, Killer Sudoku Online has a weekly greater-than sudoku. The greater-thans I’ve tried so far have been less challenging than other variants, but I’ve seen people claim it can be fiendish.
Greater-than is sudoku-like enough to come in some of the standard variants: here’s a samurai greater-than sudoku and a jigsaw (nonomino, geometric) greater-than sudoku.
A related puzzle type is futoshiki, which got off to a bad start in the Guardian.