I've made some sudoku variant puzzles that I'm sure must have been done before, although I've never seen them. (I like creating customized variants instead of regular sudoku because there are any number of automatic sudoku generators out there.) One of them I am calling "Sequence Sudoku," which is what it sounds like. It''s a regular sudoku grid with linear outlined areas. Each outlined area contains a series of numbers in sequence, with no numbers skipped. I've seen the similar "worm sudoku," but those puzzles allow numbers to be skipped. Not all cells are outlined (don't know if it's possible to make one with all the areas outlined; maybe it is) and all the clues are in the non-outlined areas. Very few clues are actually necessary because of the logical constraints of the sequences. I don't make these symmetrical because a lot more can be done with asymmetrical puzzles.
Another is Mountain Peak Sudoku. It's like greater-than/less-than sudoku, but more like a hybrid between that and regular sudoku. Circled cells are "peaks" whose numbers are greater than those of adjacent cells (not counting diagonals, and including cells in other regions). The peaks are placed reverse-symmetrically on the board, as are the clues.
I have some half-a-dozen variations, all with standard 9x9 grids. I haven't branched into different grid types except for Sudo-Kurve, which I have seen in the book Sudoku Masterpieces by Wei-Hwa Huang and Thomas Snyder. Has anyone gotten this book?
What unusual or bizarre variants do you like? (I mean besides the usual killer, diagonal, jigsaw, and samurai variants.)