Flours/Meals
Flaxseed (linseed) meal is a favorite flour of mine. While I prefer golden over brown, but they are both quite tasty. ¼ cup (28 g) has over 6 g of omega-3s and only 0.5 g of net carbs, as most of the carbs it contains are fiber. The coarser texture and nuttier flavor of flaxseed meal make it more suited to types of recipes that would be fitting for whole wheat flour: pancakes, quick breads, muffins, crackers, flatbreads, and yeast-risen breads. In my experience — albeit, limited, as I don't generally bake sweeter things like cakes and cookies — it is less suited to desert-type recipes, as the texture seems out of place. Note that the soluble fiber it contains tends to want to hold onto moisture for dear life and not let go. In cases like that, it works well to pair it with other "drying agent" type flours like whey protein powder and egg white protein powder (see below). It doesn't make a good breading or filler (i.e. for meatballs or meatloaf) because the soluble fiber can result in a slimy mouthfeel. Flaxseed can also be cheaper than other types of flours (e.g. almond flour).
Almond meal or almond flour is another low-carb flour. It has a more neutral flavor and fine texture that lends itself to desert-type recipes (cookies and cakes). Slightly higher in net carbs than flaxseed and also more expensive. Almond flour is relatively similar to flaxseed in its absorbency, at least when compared to some other flours (i.e. coconut flour). I've found 4 Tbsp of almond flour + 1 Tbsp of psyllium husk powder to be equivalent to 5 Tbsp of flaxseed meal.
Coconut flour is another low-carb flour. It has a fine texture and sweeter flavor, but it is extremely absorbent. Many recipes combat this problem by simply throwing more eggs at it, with the unsurprising consequence that the baked goods are notoriously eggy. A better solution is to mimic the function of an egg with, say, 40 g of a low-carb milk (almond milk, buttermilk, etc.), and a small amount of psyllium husk powder (to help retain moisture and contain gasses), and unflavored whey protein (to help provide the structure an egg provides).
While not a viable flour in and of itself, Vital Wheat Gluten is a staple in making low-carb yeast-risen breads (yes, they are possible). An example recipe is https://www.reddit.com/r/ketorecipes/comments/8xe3np/ultimate_keto_bread_v20/. (In baker's percents, that recipe's about 26% flaxseed meal, 26% oat fiber, and 48% vital wheat gluten. It also borrows techniques used in gluten-free baking like using xanthan gum and eggs to help improve structure). I've been able to adapt that same bread recipe to flatbreads, pizza, and tortillas.
Wheat bran and wheat germ are lower-carb parts of the wheat kernel that when used in small amounts can provide some wheat texture and flavor without all of the carbs.
While not really a flour, oat fiber is a useful no-calorie, no-carb filler. It's used in the yeast dough recipe above as a filler to lower the calorie count somewhat (as flaxseed meal is fairly high in fat).
Whey protein powder is a low-carb "drying-agent" type flour ingredient. It can help boost the protein content and provide structure. It also helps dry out ingredients like flaxseed meal. For example, I developed a low-carb buttermilk pancake recipe that uses 60% flaxseed meal and 40% whey protein powder. (100% flaxseed resulted in pancakes that had set on the outside, yet were raw on the inside). Unflavored whey protein powder also makes a great breading component for fried foods.
Grated parmesan cheese also makes a great breading component. (I usually dredge in unflavored whey protein first, then egg, then parmesan cheese). Once it's fried, you'd never know it was parmesan cheese. Parmesan also makes a good thickener in savory type dishes.
Other flours I've heard of but haven't used are lupin flour, resistant wheat starch, carbalose flour, wheat protein isolates, and carb-quick.
Thickeners/Gelling agents
Xanthan gum, psyllium husk powder, guar gum, are all low-carb and useful in helping retain moisture and gasses in baked products. Xanthan gum, unflavored gelatin, and vital wheat gluten are sometimes used in cookies to provide chew and/or prevent a crumbly texture. Psyllium husk powder works well to retain gasses in quick breads and yeast breads. It needs to be used in small amounts or otherwise combined with drying-type ingredients to allow the middle of baked goods to get fully baked. For example, https://www.dietdoctor.com/recipes/the-keto-bread uses a combination of almond flour, psyllium husk powder, and egg whites to create a quick-bread dinner roll.
