These are some FREE substitution/ replacement/ alternative software available online. I am already using some of them at work. Although they can neither substitute nor replace 100% of the commercial ones, I think they are sufficiently good for basic/ intermediate level non-professional users.

The best part is, they are absolutely FREE. It is as good as it gets for not paying anything for something that may otherwise costs a few thousand dollars. Even better still, many of them have comprehensive online learning resources, forum, and user communities. So switching is really quite a breeze!

P.S. the descriptions are cut-and-paste from the respective websites.

Adobe Illustrator

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Inkscape is an Open Source vector graphics editor, with capabilities similar to Illustrator, Freehand, CorelDraw, or Xara X using the W3C standard Scalable Vector Graphics (SVG) file format. Supported SVG features include shapes, paths, text, markers, clones, alpha blending, transforms, gradients, patterns, and grouping. Inkscape also supports Creative Commons meta-data, node editing, layers, complex path operations, bitmap tracing, text-on-path, flowed text, direct XML editing, and more. It imports formats such as JPEG, PNG, TIFF, and others and exports PNG as well as multiple vector-based formats.

Adobe Photoshop

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Paint.NET is free image and photo editing software for computers that run Windows. It features an intuitive and innovative user interface with support for layers, unlimited undo, special effects, and a wide variety of useful and powerful tools. An active and growing online community provides friendly help, tutorials, and plugins.

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GIMP is the GNU Image Manipulation Program. It is a freely distributed piece of software for such tasks as photo retouching, image composition and image authoring. It works on many operating systems, in many languages.

GIMPShop dot Net

GIMPShop is a free Open Source image editor that is similar to the popular Adobe Photoshop. Specifically GIMPShop is a version of the GIMP that has been edited to be more user-friendly for Photoshop users.

Adobe PageMaker
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Scribus is an open-source program that brings award-winning professional page layout to Linux/Unix, MacOS X, OS/2 and Windows desktops with a combination of “press-ready” output and new approaches to page layout.

Adobe Acrobat (Creating PDF Documents)

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PDFCreator is a free tool to create PDF files from nearly any Windows application.

Macromedia Dreamweaver

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Finally! A complete Web Authoring System for Linux desktop users as well as Microsoft Windows and Macintosh users to rival programs like FrontPage and Dreamweaver. Nvu (which stands for “new view”) makes managing a web site a snap. Now anyone can create web pages and manage a website with no technical expertise or knowledge of HTML.

Microsoft Office

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OpenOffice.org is a multiplatform and multilingual office suite and an open-source project. Compatible with all other major office suites, the product is free to download, use, and distribute.

WinZip

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7-Zip is a file archiver with a high compression ratio.

(Others)

Recording/ Sound Editing

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Audacity is free, open source software for recording and editing sounds.

RSS

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Feedreader is a FREE RSS news aggregation solution that provides robust, state-of-the-art features in an intuitive, user-friendly environment.

Podcast

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This program lets you create your own custom online audio anytime, anywhere. Really. If you want to listen to podcasts, this program is for you. Juice is the premier podcast receiver, allowing users to capture and listen to podcasts anytime, anywhere.

3D Graphic

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Blender is the free open source 3D content creation suite, available for all major operating systems under the GNU General Public License.

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Art of Illusion is a free, open source 3D modeling and rendering studio. It is written entirely in Java, and should be usable on any Java Virtual Machine which is compatible with J2SE 1.4 or later.