Even though open source software isn't a new thing anymore, it's still revolutionary. Not only because it allows anybody to see, use and alter the source code of the software as such, but also because it allows for free distribution and redistribution of the software.
And even though there are some rather famous pieces of software that are based on this principle - such as Linux (the operating system) and Open Office (the open source alternative to Microsoft Office) - most designers don't know too much about open source alternatives for all the expensive software they are using. There are all kinds of ideological arguments for going towards open-source software. And it's probably just as important an argument that the open source software is free for use - which is a big advantage over the rather expensive licenses for programs like photoshop, autocad, 3d studio and all that.
So what exactly are the open source alternatives for the most common software packages used by designers? I'm purely listing actual design software - all the software that is useful in running an office (backup software, FTP-software, financing software and all that) is left out, focussing solely on open source software that you'd actually use in your design and presentation process.
Archimedes is a open source clone (well, kind of...) of Autocad 2D.
AvoCADo is an open source 3D CAD program. I'm not too sure about the quality, though.
Blender is an open source 3D creation/animation suite. Consider this among the best alternatives to 3D Studio, Maya, or whatever 3D program you might be thinking about.
BRLCAD is a CAD-program which works with solids modeling - like a quick and easy way to put a preliminary sketch in a 3D model.
Cute PDF is, technically speaking, not open source, but it's free for use and a pretty good alternative to Adobe PDF creator or Adobe Acrobat.
Cademia is a basic 2D CAD drawing program.
Dia is loosely based on Microsoft Visio, and is a tool for the creation of all kinds of diagrams.
Gimp, short for GNU Image Manipulation Program, is a program for photo manipulation, image retouching and all that. It could be seen as an alternative for Adobe Photoshop.
Gimpshop If you are considering going towards an open source software setup, but you know your way in photoshop too much to feel comfortable moving towards Gimp, Gimpshop might be the program for you. It's a version of the original GIMP, created to make everything look as much like photoshop as possible.
Imgv is a image viewer with loads of features. An alternative for programs like ACDSee?
Inkscape is an open source vector graphics editor. Consider this as an alternative for Adobe Illustrator and CorelDraw.
Jahshaka is a movie-editing tool. It pretty much has all the standard features that one can find in programs like Adobe After Effects.
Kerkythea is a standalone renderer, with high-quality materials and light settings.
Moonlight 3D is aiming to be a extensive, flexible modelling, animation and rendering tool. It could be a good alternative for the most commonly used 3D software packages.
Open Office is, as noted above, an open source alternative for the Microsoft Office suite. It pretty much does everything that programs Word, Excel, Powerpoint and such do.
Pixie is an open source render engine for creating photorealistic images.
Qcad is a simple, 2D CAD drawing tool. It's not totally refined, but it's (in my eyes) the best alternative for Autocad - but it's still lacking some key features.
Scribus is an open source program for page layouting. Consider this a great alternative for Adobe Indesign, Quark Xpress or those kinds of programs.
Skencil is another vector based drawing software program.
Sketchboard is a 3D CAD-tool for converting sketches into simple polygons. It recognizes simple drawings and converts them for further development.
Xara LX is an open source version of Xara Xtreme - which is a graphics software package. It's (for now?) only for Unix/Linux kinds of operating systems, and I believe they are working on a OS X version.
And if you are looking for an open source alternative for specific software, try the osalt.com website - which lists open source alternatives for almost every possible software.
Some more useful links could be: SourceForge - a website with tons of downloadable open source programs or entirelyopensource.com - listing all kinds of programs and open source news. Oh, and Wikipedia.org has a big list of open source software packages.