Used

Balsamiq - simple fast to start and design forces you to focus on the User Experience over the graphics design.

...

iRise - more powerful Axure but when I looked at a long time ago it was a bit of a beast for non developers.

To Evalute

HotGloo  - is all about interactive prototypes (still to evaluate) - websites has good explanation and diagrams on wire-framing.

MockFlow - mockflow creates a suite of tools for collaboration and reviews of the designs. Still good at simple wireframes they also let you add interactions.

Justinmind - yet to evaluate.

References

Starter Revew - http://www.quora.com/Which-web-based-wireframing-tool-do-you-recommend-Balsamiq-vs-Mockflow-vs-UXpin-etc