Dusts

Dusts are gained from macerating their respective ores. With the exception of coal, which only returns one dust per piece, all ores when macerated will give 2 dusts of their type. Ingots can be macerated down again to form one dust. A bug occassionally results in the production of just one dust from iron and possibly other ores.

Usage
Dusts, apart from Coal Dust and Clay Dust, can be smelted into ingots, allowing for 2 ingots per ore. Dusts may also be used in a variety of recipes for machines and items.

Iron Dust Heap
You can craft Iron Dust Heaps from copper and tin dust, two of which can be crafted into iron dust.

Clay
Note that Clay Dust is different from Clay.

Piles of clay as extracted from the ground can not be macerated. Four piles of clay can be crafted into a block of clay which can then be macerated into two Clay Dust.

Clay dust is used to make construction foam which can then be compressed into CF pellets to be used in a CF Sprayer or a CFPack.

Nether Ores
Nether ores can, like any other ore, be macerated. Unlike when smelted, you receive two ore blocks instead of one - effectively quadrupling your mining yield from the nether.

While highly profitable, Gold and Iron Nether ore is bugged in SMP and refuses to be macerated properly. Instead, it displays as macerated until the player tries to remove it, whereupon it returns to the input slot and turns back into ore.

Nether Redstone ore can be macerated for a guaranteed drop of five units of Redstone dust.

Coal to Diamond
Coal Dust can be turned into Coal Balls like this.

From there, you can then use a Compressor, to compress the Coal Balls into Compressed Coal Balls

After that, you can then turn 8 Compressed Coal Balls and 1 Obsidian, into a Coal Chunk

Just compress the coal chunk, and you get a diamond!