Balsamiq

Designed to make wireframing as quick and as user-orientated as possible, Balsamiq is a simple WYSIWYG tool for sketching out wireframing ideas. This is really handy for experimenting with new page layouts, or for reworking your user interface. It’s a lightweight tool that is quick to use, presenting either sketch or clean-skin mode for in-house development. The perfect tool for providing a basis for brainstorming and UI feedback, Balsamiq comes in at $12/month, or a one-time purchase of around $80.

http://www.balsamiq.com