Puzzle Japan
Puzzle Japan is the puzzle website for Nikoli, a popular puzzle publisher in Japan that popularized sudoku puzzles. Their website offers eight different logic puzzles, including sudoku and kakuro, available in online and printable formats. Many of the puzzles involve building walls or boundaries, an activity reminiscent of the game of Go. One of my favorite puzzles is Nurikabe, in which you fill in cells to create islands.
From December 31st to January 5th, the members section of Puzzle Japan is free. Be sure to check it out.