I just went through the process of reviewing these and I thought 3 stood out for my purposes. Note that I did not want to pay a monthly fee. I am a single wannabe designer / developer that is just getting started, so keeping expenses down is quite important. The 3 that I really thought stood out were Balsamiq, Wireframe Sketcher, and Iplotz. All have trials and are quite approachable and easy to use. I don't really think you can go too far wrong with any of them.
Note that my least favorite of these 3 was Balsamiq. The monthly version includes a multi-page project feature, but the standalone version does not. This was a bit of a deal breaker for me even though overall it was probably the best product. Lack of a single feature knocked it down for me.