Last Candidate
Beginner Places a digit Used in: Sudoku, Killer Sudoku, Calcudoku, Futoshiki
After previous candidate eliminations, a cell may be left with a single remaining possibility. That last candidate is forced, so place it — the earlier cleanup has already ruled everything else out.
How to spot it
Keep your pencil marks current and watch for a cell whose candidate list has shrunk to one.
What you can conclude
That last surviving candidate is the answer.
Worked example
After a naked pair clears 4 and 8 from a cell that read {4,6,8}, only 6 remains — fill it in.