Be aware that the thread is almost 6 years old, thus most apps changed or are completly abandoned !
Xnormal might be the best choice, since its free and offers quite a lot of features (with a rather complicated interface, but its free)
http://www.xnormal.net
Mapzone offers a nice wide variety of noise patterns as a general start for masks and textures.I use it for quite some time to do natural phenomena like frozen surfaces, dirt, stone and more..
A more professional version of this concept is Filter Forge
http://www.filterforge.com/ which costs quite lot
I find it sometimes to hard to get a final solution out of these node based procedural setups, but they provide infinite variations for basic overlays and masks.
Gimp - Free Linux and Windows
There are some good free software packages out there !
Gimp improved much more over the years since the first install i had 10 years ago
http://www.gimp.org/windows/
Inkscape
For interface stuff and vector work i highly recommend Inkscape which proved very stable and usable for some projects.
http://inkscape.org/
For 3D Sculpting there is Sculptris
http://www.sculptris.com/index.html
a very small app, but surprisingly well usable. Its on a hold since the author got a job at Pixologic (developer of Zbrush, top software for sculpting) but its is still available.
If one is having some budget i would suggest to give 3d Coat a try, its offering many good features and is also quite fun to use.
For 350 bucks you get sculpting, texturing, uv unwrapping, retopologizing, modelling, rendering and all with CUDA based acceleration and some highend features like Disney's Ptex algorithm..
http://www.3d-coat.com/what-is-3dcoat/ there is also a demo for tryout.
A little side note: This is also a one-man-army product, a single guy coded the software.. pretty cool.