most of mine have already been mentioned, but i'll add my 2 cents anyway.

Matmatic - no better tool for creating materials.  i can create multiple colors of one type of material in a very short time, and create materials for a whole scene.

because of the above, i no longer need ShaderSpider (i create my own material collections), but i used it all the time when i didn't use Matmatic.

Blender is by far my most useful tool.  the sculpting, modeling, cloth sim and fluid sim tools are really useful to me.  i highly recommend it.  other 3d tools might be as useful, but even if i've heard of them shipping with sculpting and cloth tools, i'm not as sure about built in fluid sims.  it also does UV mapping, but not as powerfully as some UV mapping tools, and you can do projection painting and such, but again, not as powerfully as some other  tools.

the online scientific calculator at http://web2.0calc.com/.  there's some crazy stuff i do with materials that have needed it, but it's just as important to work out scale issues when modeling.  even if you're just eyeballing one part of something, you might want to calculate the rest of a model based on that original size.

Wacom pen & tablet.  absolutely indispensible.

mechanical pencil, eraser, sketchbook, and notebook. the sketchbook is useful for sketching out ideas, thinking about how something looks at different angles, and occasionally jotting down settings or other numbers.  the notebook is good for recording what i'm doing and why, so i don't keep forgetting solutions to problems.

Cr2 Editor.  really, really useful for fixing stuff like references to textures.  works on everything from character files to props to materials.  iirc, it's necessary to get the version EnglishBob links to because there's a newer one that messes up your files.

since i'm still on P6 (hopefully not for too much longer), bagginsbill's GenIBL tool.  

Photoshop & Fireworks. Photoshop is great for photo editing and painting.  since Fireworks is vector based, and has lots of patterns (which you can add to), textures (which you can also add to), variable strokes, and other features, it can do pretty much everything i've seen listed as a StitchWitch feature.

tools i would like to acquire so i can do things i can't now are Zbrush and Vue.  Zbrush for certain types of folds and such in clothes, and Vue for outdoor scenes.