I was hunting badly for the best screenmockup or prototyping softwares available for several years. A tool which has tons of drag-n-droppable UI components, categorized, easy to use, light weight, are the features I was looking for.
Balsamiq Mockups is a rich mockscreen creator app which has tons and tons of UI components available (If you still want more, there are options import components! ). This app is available in Online(free!) as well as in Desktop version(79$), and other versions for Confluence, JIRA, XWiki.
I had previously reviewed a portable screen mockup software, Pencil, but Balsamiq Mockups definitely stands out as it is powerful and fully loaded compared to its alternatives. This app is loaded heavily with 75 built-in UI controls and more than 100 icons. Additional controls can be downloaded from MockUpsToGo Site which has many user contributed controls which can be imported to our editor and reused.
MockScreen Controls
You can try out the online free version of this app but it has few limitations
- You need active Internet connection (Obvious)
- Cannot work on multiple mockup screens simultaneously
- Insert Images in Mockup components.
- Change the default Comic Sans font.
For full fledged features buy the downloadable version or try the demo desktop version. Adobe Air is required for running the Desktop version
Samples
Mock Site
Real Site
This is the screenshot of my previously decommisioned site http://31nst31n-In-Me.com (Click to view it larger)
(Update: Author was benevolent enough to provide me with full version of this app for testing, so I was able to insert images in the my sample mockup below and so was I able to explore all the features…)
Checkout the sample mockups created from this page.
Demo
Go to Balsamiq Mockups
Download Desktop Version : 
