Shikaku:
Rules - Divide the grid into rectangles along the grid lines so that exactly one given circle is inside each rectangle and so that the number in that circle indicates the total area of the rectangle. See also here.
Masyu:
Rules - Draw a single closed loop in the grid passing vertically and/or horizontally through the centers of the white cells. The loop must pass through every cell containing a circle. When passing through a black circle, the loop must make a 90 degree turn and must pass straight through the first cells encountered on each side of this turn. When passing through a white circle, the loop must go straight through the white circle, but must make an immediate 90 degree turn in one of the adjoining cells. See also here.
