Powerful Web sketch tool Balsamiq Mockups

Balsamiq Mockups is an easy-to-use Web sketch drawing tool. It is easy to use and can easily draw a Web interface prototype. With this feature, you can easily create a cute interface, you can see it.

That's so cool!

The software is good but not free.
However, the author is quite honest. There are a variety of methods for getting keys for free on the home page. You can log in and see if there is a method that suits you.
URL: http://www.balsamiq.com/
Online Demo address: http://www.balsamiq.com/products/mockups
Trial site: http://www.balsamiq.com/demos/mockups/Mockups.html