Creek is a puzzle originally from nikoli; the best external link I can find is here. The object is to shade in some squares of the grid. Some numbers inside circles are in the corners of grid cells, which tell how many of the grid cells it touches are shaded in. The unshaded cells in the finished grid must form a single contiguous region. There is no restriction on unclued squares; they may be shaded or unshaded. The solution is unique.
Below is an example puzzle and its only solution.