Balsamiq - simple fast to start and design forces you to focus on the User Experience over the graphics design.
Axure - cost effective and more powerful interactive prototyping.
iRise - more powerful Axure but when I looked at a long time ago it was a bit of a beast for non developers.
- 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.
2014 Lineup (looks like real users) - http://www.woorockets.com/8-premium-wireframing-mockup-tools-for-designers-in-2014/