Some *hard* Sudoku variations that have appeared in Internet Puzzle Competitions:

From PQRST 11, puzzle #5:

From IPST 2005, 2nd round, #6

Every figure contains digits from 1 to 9. Place these figures into the grid so that all digits in every row and column will different.

From IPST 2004, 1st round, #4

Put numbers from 0 to 9 into the cells, so that every row, every column and every marked region have not repeating number.

PQRST stands for Puzzles Quarterly Rate Solve Ten. As it's name implies, it is Quarterly, competitors must rate the puzzles as well as solve them -- and there are ten puzzles each time. Users have 7 days to submit answers. There is no cost, no prizes other than getting your name on the list of high scorers. All past puzzles and answers are online.

PQRST 14 starts on August 6th, Saturday at 20:00 (GMT+02).

IPST stands for Internet Puzzle Solving Test. It is also quarterly, has no entrance fee or prizes other than bragging rights. Again, you have 7 days to solve. This contest is run by Diogen, a Russian puzzle club, so the translations into English can add an additional layer of compexity to solving. All past puzzles and answers are online.