Wireframing is an often forgotten concept that is essential to the creation, planning and analysis of websites as well as applications, documents, graphic design pieces, architectural designs, software and storyboards. Wireframing applies to any activity that involves a creative process that will determine a final product or response of any kind.
We will be focusing on discussing wireframing in terms of website design mostly.
So what do we mean by a wireframe for a website? It’s simply a proposal of a graphic-free layout for the website. The order and continuity of the fundamental elements of the interface are laid out before any web design or artwork is completed.
An example of a website wireframe. Graphic from www.totheweb.com

An example of a sketched wireframe. From Paper prototyping: the fast and easy way to design and refine user interfaces by Carolyn Snyder

This allows a clear and distinct connection to be made between the web designers and web engineers. Foreseeable problems can be sorted out in possible trouble spots, and the navigation flow is determined by the usability and relationship within the elements themselves. This process is not easy and requires a lot of detailed attention and thought. Your project’s success relies heavily on the amount of effort put into it during this initial yet crucial stage.
There are various tools available for wireframing. The most rudimentary method, called Lo-fi prototyping, is paper prototyping, which began in the mid 80s and gained popularity in the mid 90s when IBM, Honeywell and Microsoft implemented these methods in their product design process.
Lo-fi wireframing programs: Visio, Balsamiq, InDesign, Illustrator, Photoshop, and OmniGraffle.
Hi-fi wireframing programs: Axure, ProtoShare, Justinmind, and Irise.
You can use HTML/CSS Wireframing, Wireframing Software or simple sketching. It really depends on the level of complexity you need for your project to achieve.
Wireframing and Prototype PDF Downloads and Templates:
The Information Architecture Institute
http://iainstitute.org/tools/
This project attempts to disseminate new IA tools for the community to share. It includes fluidIA, Shades of Gray: Wireframes as Thinking Device, Content Value Analysis (CVA), Content Inventory/Site Map Tool, Task Analysis Grid, Site Map Templates, Omnigraffle Stencils, etc.

Designer Toolbox: Sitemap and Wireframe Templates
http://www.designerstoolbox.com/products/forms/100129/
Available for 15 bucks, this complete includes tested templates that will allow you to create client ready professional website wireframes.

User Goals: HTML Wireframe Templates
http://www.usergoals.com/wireframes/html_templates/index.html
These free templates available in two dimensions, 800 pixels and 1024 pixels wide, give you a good set of options for your layouts. Their website also features wireframes for invoicing, mobile, magazine, simple layouts, and intranet.

Yahoo! Design Stencil Kit version 1.0
http://developer.yahoo.com/ypatterns/about/stencils/
Available for OmniGraffle, Visio (XML), Adobe Illustrator (PDF and SVG), and Adobe Photoshop (PNG). Content includes Ad Units, Calendars, Carousels, Charts and Tables, UI Controls, Form Elements, Grids, Menus and Buttons, Mobile – General and iPhone, Navigation and Pagination, OS Elements, Placeholder Text, Screen Resolutions, Tabs, Windows and Containers.

Eightshapes Unify
http://unify.eightshapes.com/users-guide/what-you-get/wireframe-templates/
Sign up and you’ll get a download link by email.
OmniGraffle Wireframe Stencils
http://konigi.com/tools/omnigraffle-wireframe-stencils
This set of lo-fi templates for OmniGraffle offers a complete range of basic elements needed to create interface specifications.

Nokia S60 Wireframing Stencils
http://www.forum.nokia.com/info/sw.nokia.com/id/cfc7b6a4-2dc5-4c91-88a5-c35764fff8fe/S60_Wireframing_Stencils.html

Janko at Warp Speed: Free Sketching and Wireframing Kit
http://www.jankoatwarpspeed.com/post/2009/12/24/sketching-wireframing-kit.aspx
This great kit contaitns the following things: form elements, icons, indicators, feedback messages, tooltips, navigation elements, image placeholders, embedded videos, sliders and common ad banners.

Wireframing and Prototype Links:
Paper Prototyping
http://www.paperprototyping.com/
Wireframes Magazine
http://wireframes.linowski.ca/
ToTheWeb - Wireframe Sample
http://totheweb.com/learning_center/website-wire-frame.html
W3Avenue – Guidelines, Tools and Resources for Web Wireframing
http://www.w3avenue.com/2010/02/03/guidelines-tools-and-resources-for-web-wireframing/
I <3 Wireframes Flickr Pool
http://www.flickr.com/groups/ilovewireframes/pool
Anthony Armendariz
http://anthonyarmendariz.carbonmade.com/
Wireframing and Prototype Books:
Paper prototyping: the fast and easy way to design and refine user interfaces by Carolyn Snyder
http://www.amazon.com/Paper-Prototyping-Interfaces-Interactive-Technologies/dp/1558608702/ref=sr_1_3?ie=UTF8&s=books&qid=1272143161&sr=1-3
Communicating Design: Developing Web Site Documentation for Design and Planning
http://www.amazon.com/Communicating-Design-Developing-Documentation-Planning/dp/0321392353/ref=sr_1_1?ie=UTF8&s=books&qid=1272142040&sr=8-1
Prototyping: A Practitioner's Guide by Todd Zaki Warfel
http://www.amazon.com/Prototyping-Practitioners-Todd-Zaki-Warfel/dp/1933820217/ref=sr_1_1?ie=UTF8&s=books&qid=1272142096&sr=1-1
Dynamic Prototyping with SketchFlow in Expression Blend: Sketch Your Ideas...And Bring Them to Life by Chris Bernard and Sara Summers
http://www.amazon.com/Dynamic-Prototyping-SketchFlow-Expression-Ideas/dp/0789742799/ref=sr_1_3?ie=UTF8&s=books&qid=1272142096&sr=1-3
Effective Prototyping for Software Makers (Interactive Technologies) by Jonathan Arnowitz, Michael Arent , and Nevin Berger.
http://www.amazon.com/Effective-Prototyping-Software-Interactive-Technologies/dp/0120885689/ref=sr_1_4?ie=UTF8&s=books&qid=1272142191&sr=1-4
User-Centered Design Stories: Real-World UCD Case Studies (Interactive Technologies) by Carol Righi and Janice James.
http://www.amazon.com/User-Centered-Design-Stories-Interactive-Technologies/dp/0123706084/ref=sr_1_6?ie=UTF8&s=books&qid=1272142191&sr=1-6
Information Design Workbook: Graphic approaches, solutions, and inspiration + 30 case studies
http://www.amazon.com/Information-Design-Workbook-approaches-inspiration/dp/1592536271/ref=sr_1_3?ie=UTF8&s=books&qid=1272142191&sr=1-3