Regular skyscraper: Fill the grid with the digits 1 to 9. The digits represent the height of the Skyscraper in each cell. Each row, column and 3x3-box will have exactly one of each digit. The clues along the egdes tell you how many Skyscrapers you can see from that vantage point.
http://www.sachsentext.de/en/index.htm (Pyrrhon's website?) contains 2 examples of these. I think that the difficulty of my skyscrpaer sudoku is inbetween that of the two posted on Pyrrhon's site. It shouldn't be too difficult.
Skyscraper sudoku X: as regular skyscraper, except with additional regions on the main diagonals. Once again, should not pose too much of a challenge.
The least number of givens possible on a regular skyscraper is 6. (placing 9 6 _ | 8 5 _ | 7 4 _ along the top). The least number of possible givens on a skyscraper X, on the other hand, should be larger, even though there are more constaints. If somebody were able to calculate that, that would be great.
Killer skyscraper. Another predictable variant, but one with lots of potential. This one's a medium pearl, and should be fairly difficult.
Diagonal Skyscraper: This time, the digits on the outside of the sudoku represent the sum of the number of skyscrapers that you can see along the two (or one) diagonals from that vantage point. If two skyscrapers of the same height are next to each other, you can only see the first one. This one is a good difficulty and an interesting twist. I would highly recommend it.
Even/Odd skyscraper: Shaded regions represent even numbers, unshaded regions represent odd numbers. This is another interesting puzzle, and once again, I recommend it.
Lastly, here's a difference skyscraper sudoku. The numbers represent the positive difference between the two adjacent cells.
Enjoy!
~Squirrel
