David Millar, the community section editor of Sudoku Xtra, is running a fantastic daily Advent Calendar sudoku puzzle
on his website at thegriddle.net. There's a big overall grid you print out and then each day you get given new pieces with different variant types to add into it. David's even promised postcards with custom puzzles as prizes to those who manage to complete the entire puzzle.
It looks great to me, so I'd definitely recommend taking a look.