Hashiwokakero (bridges) solver
Type in a bridges puzzle and get its answer, a check of whether it has exactly one, and the logic steps that crack it. It runs in your browser: nothing you type leaves this page.
One line per row. Type each island's number (1 to 8) and a dot for every empty square. All rows can be different lengths; short ones are padded with empty squares.
The logic, step by step
How the solver works, and what it will not claim
The solver builds every straight, clear link between islands, then repeatedly applies three rules: an island's number limits how many bridges each of its links can carry, a link is ruled out when a bridge already has to cross it, and a bridge is ruled out when it would close a group of finished islands off from the rest. If those rules finish the board, it prints the steps. If they stall, it tries one link at a time to count the solutions, and says so, because a puzzle that needs trial and error is not one you could solve by deduction alone. It counts up to two solutions, so the verdict is exactly one, more than one, or none. It stops after about two seconds on very hard boards and tells you it gave up, instead of guessing.
The same engine checks every Causeway daily puzzle, together with a second, independently written counter. For the techniques by hand, read the guide to solving bridge puzzles.