QGIS looks very interesting - have you used it on a Mac? I'm wondering how solid it is. I don't care for GIMP because I crash it too easily - and barely tolerate Sketchup on Mac because of that. Inkscape is less frequent.
It should be mentioned that Pixelmator is a Mac only app - it looks interesting, especially as it supports (again) Quartz Composer filters. It doesn't support Adobe or other plugins, though, IIRC.
Like many of the Mac products, one must have a version that supports it (most require 10.5/10.6 and above due to Apple's architecture and OS changes when Job's returned and brought the NeXT technology with him).
Adobe Elements is a lower end product from Adobe - I haven't used it myself, so can't say how good it is for actual creations.
Blender should also be tossed out there - its free and its powerful. But its learning curve is even steeper than the rest, IMO.
Inkscape, Sketchup, GIMP, QGIS are all multi-platform (Windows, Mac and all but Sketchup also support other UNIX platforms, I believe).
For Windows - http://www.irfanview.com/ is free and supports numerous plugins. It is an awesome viewer and really good at converting and manipulating images - its Lancoz scaling method is generally better than what Adobe offered (at least in older versions). It even has a limited Paint feature built right in.
Not only that - I don't think I've EVER crashed that program (been using for ~ 15 years!). I can't say that of ANY commercial product I've ever bought.*
Also for Windows - Paint.NET is useful and free. It supports transparency, filters, and layers, etc.
I should also mention, on any platform, from virtually any text editor (even EDLIN or VI):
- One can write postscript and make PDFs. PDFs are fundamentally just pre-processed postscript. Postscript can be send direct to a printer or opened and converted with the free Ghostscript or, expensive though older versions can be found cheap, Adobe Acrobat distiller. On Macs, one can also distill for free from the command line (though, I think PDFMark support may be limited).
For images (i.e. not PDF vector files) www.povray.org scene descriptions can also be written - yielding awesome results with the right patience and skills with this free raytracer+ that has been around for probably 20+ years.
If you can find it, there is also the older free BMRT which was ILM RenderMan compatible and actually used by ILM before PRMan incorporated ray-tracing... several free projects are around, though I haven't found any feature complete.
The above list, however, is not for those who don't really want to invest in learning new stuff - if you just want to get a result with minimal neural expense - stick to the GUI apps...
*The free programs do, generally, accept donations!