Your standard "connect the pipes" game. Click on a section of pipe to rotate it counterclockwise. The puzzle is solved when all pipe sections are green.
One option that would be very helpful would be if you could right click on a square to change the background color (say, to gray), so you could mark which tiles you are sure are in the correct orientation.
The other "must have" feature would be larger puzzle sizes.
Keep up the great work and let me know if I can help with, or take over, the development of future versions.