preamble
Every software development company needs special software to make prototypes of user-interface,
and it doesn’t mater if we do a new system or redesign of old one.
Often people say: they don’t need to prototype or they have special person for such tasks. Don’t believe them, it’s task of developers as well as writing tests.
If you still think differently about prototyping read famous book of Alan Kuper
“The Inmates Are Running the Asylum” : http://www.amazon.com/Inmates-Are-Running-Asylum/dp/0672316498
let’s do prototyping
On Interaction Design Association (IxDA) [http://www.ixda.org/] discussion forum we can find that people use almost any tool to do prototyping: ACD Canvas, Microsoft Word, SmartDraw, Inspiration, Microsoft Power Point, etc.
First of all forget about software for graphic manipulations like: Adobe photoshop or Corell draw, then about software that don’t have any connection to prototyping at all. With such software you will lose your time, and will think that prototyping is hard work for designers.
There are big amount of commercial software to prototype user-interfaces.
As famous commercial software I can note:
Axure: http://www.axure.com/
Adobe inDesign: http://www.adobe.com/products/indesign/
Omnigraffle: http://www.omnigroup.com/products/omnigraffle/
Pidoco: http://pidoco.com/en
great and free applications
Pencil
site: http://www.evolus.vn/pencil/
* Multi-page document with background page
* Good text editing with rich-text supports
* Exporting to HTML, PNG, Openoffice.org document, Word document and PDF.
* Installing user-defined stencils and templates

Mockingbird
site: http://gomockingbird.com/
* Multi-page document
* Good text editing
* Exporting to PNG, and PDF.

Balsamiq Mockups
site: http://www.balsamiq.com/products/mockups
* No multi-page document, and background
* Good text editing
* Exporting only to PNG
* Installing of user-defined elements only thru importing them as a pictures
additional feature: integration to popular project management tools

Still can’t select something
As start point try Pencil: http://www.evolus.vn/pencil/
It should meet all you basic expectations and save your money.
If you prefer to have commercial software that contain all features you ever wanted then try Axure:
http://www.axure.com/