As a software developer, I am certainly no enemy of commercial software. However, the quality of some of the free and open source alternatives has been giving their commercial competitors quite a run for their money. This is especially true for the casual or occasional user. It would be extremely difficult to justify a complete set of professional-level commercial tools just to support a hobby, but when the price is $0, the question is more one of learning curve and disk space.
Adobe has always had a large market share of the paint and drawing markets with Photoshop and Illustrator. They invented the PostScript page description language and the closely related PDF file format. They are the current owners of the high-end publishing system FrameMaker, and recently became the owners of the widely used Shockwave and Flash tools. For each of these niche products, a slew of "almost as good" commercial competitors have also been around for a while, but the open source community has been a lot slower to catch up.
Here are a few free alternatives that should be on the radar:
The GIMP is a paint tool that has overcome its anti-Windows roots to become a widely used and extremely powerful competitor to Photoshop. I haven't personally used it (when I had the chance to adopt it instead of Photoshop, its developers were still exceedingly hostile to the idea that it would ever run on Windows, so I bought Photoshop instead) but reviews and testimonials make it pretty clear that it must be taken seriously. If I were new to the market today, I would strongly consider buying Lightroom from Adobe for its RAW file handling and lightweight DAM capabilities and installing the GIMP for use on larger editing tasks such as stubborn corrections and montage.
Inkscape is a vector graphic drawing tool that set out to be the both the best possible drawing environment it could and the best example of a full-featured editor for the SVG file format. It is well on its way to succeeding on both fronts. I had tried using Corel Draw in the dark ages (under Windows 3.0, IIRC) and never got hooked. Inkscape lacks the easy templates and shapes of Visio, but then it isn't primarily a diagramming tool, and Visio was never intended to be an art and illustration tool.
Scribus is clearly aiming to become the best available professional publishing tool. This is not for casual users. But if you had a budget and would be considering FrameMaker, taking a good hard look at Scribus might pay for a fair amount of pizza and coffee. I have used FM (it was standard at a customer's site in 1988) but I haven't personally used Scribus (yet) so I'm basing this impression on reviews and testimonials. FM in 1988 was complicated, huge, heavy, expensive, and exceedingly reliable when producing huge technical manuals. FM today is more complicated and no less expensive. Of course, if having a GUI is not critical, then the clear winner over both is probably LaTeX. A good distribution that runs well under Windows is free from MiKTeX.
Blender is one other tool that deserves a mention, although I have not had the chance to even give it a test run yet. It describes itself as a "free open source 3D content creation suite." It seems to have an extremely complete set of modeling, rendering and animation features.