#1 myBalsamiq

myBalsamiq website | myBalsamiq demo

Cost: Starter Plan – $24/month (10 Active Projects, Unlimited Mockups, Unlimited Users, 50% off Desktop app, SSL Security)
Platform: web, desktop
Export options: png, pdf

Online preview
Real time collaboration and live preview
Versioning
Comments
Community
A lot of useful components
Sketchy style

Possibilities: 
Design: 

#2 UXPin

UXPin website

Cost: Freelancer’s Friend – $27/month (10 projects, 2 users, Paper prototypes recognition, 10% discount on UXPin Kits, Unlimited storage)
Platform: web
Export options: html

Online preview
Real time collaboration and live preview
Versioning
Comments are separated from mockups
Master elements
Nice interface

Possibilities: 
Design: 

#3 justproto

justproto website | justproto example

Cost: Plus – PLN 149 / month (12 collaborators, 25 projects, 5 GB server space)
Platform: web
Export options: html

Online preview
Real time collaboration and live preview
Comments
Master elements

Possibilities: 
Design: 

#4 Axure

Axure website | Axure example

Cost: $589 per license
Platform: desktop
Export options: png, html

Online preview
Versioning
Master elements
Interactive
Word specification generator

Possibilities: 
Design: 

Comparison:

Name myBalsamiq UXPin justproto Axure
Cost $24/month $27/month PLN 149 / month $589 per license
Platform web and desktop web web desktop
Export options png, pdf html html png, html
Online preview yes yes yes yes
Real time collaboration
and live preview
yes yes yes no
Versioning yes yes no yes
Comments yes yes yes no
Community yes no no no
Master elements no yes yes yes
Possibilities 3 2 2 3+
Design 2 3 2 3