If you have a vision that you want to share with minimal effort (and you can’t draw like me), I highly recommend Balsamiq. It makes mockups as easy as can be. All of the most popular components and controls are available which easily snap together. The full version is $79, but if you click Try It Now you can use the free web version. The free version includes many controls and lets you export to png – it even lets you save and come back. Enjoy.
Mockups with Balsamiq