If you make websites, you start with a pen and a paper to write and sketch. But at some time you need to prototype or show the grid, styleguide or design to the customer. Then you have tools like wireframes.

[rant]
Mind you, not photoshop. Repeat after me, "only print designer use photoshhop."

If your webpages are all A4/letter size, you should have made a book.

If all webpages can be printed like the photoshop file can, you should mail PDF's.

Websites are interactive and not bound to A4 size. Ans should not be made in tool with half pixels!
[/rant]

One of the best online tools I have seen recently is Easel, feels like "real" app with the advantages of being able to work everywhere, with multiple people and share results fast, like all webapps ;-)