When you need to quickly throw in a prototype of a website or application, use any tool from this collection.

Balsamiq

Quite a simple and intuitive resource, there is a trial version with reduced functionality, you will have to pay for a full-fledged one.

Axure

This is an ideal tool for designers and project managers, but like the previous service, you have to pay, although there is a free trial version.

Mockflow

A handy tool with an uncluttered and intuitive interface.

Moqups

You can create not only site prototypes, but sitemaps, diagrams, and more.

POP

This is a smartphone app. Paper and pencil are all you need to create an interactive prototype.

Invision

A powerful online service with a huge range of features. You can make changes remotely.

Wireframe.cc

This is perhaps the simplest service, a find for quick sketches, very primitive – but no less attractive from this.

If we forgot some service, then write it in the comments!