I can confirm that +carry weight, fortify alteration/destruction, better prices, +health, and +mana all stack. I have boots and gloves of +37 (I think) carry weight, and they stack additively to 374 total (from 300 base), and stack with my Voldun mask which adds another 20. Fortify alteration/destruction are additive, so four 25% mana reduction enchants stack to 100%, or zero mana cost. Voldun mask reduces prices, then equipping the Amulet of Zenithar reduces prices again, though I haven't checked to confirm it's additive (i.e., it should be 20% + 15% [I think] is a 35% reduction, which is close, but I haven't confirmed it). +health and +mana stack additively.
I know +weapon skill enchants stack, but I haven't really played with them. I believe they stack additively, then the combined percent is compared to the weapon damage. So if you have an improved weapon with perks and stuff, and it's sitting at 50 damage at a skill level of 75, then you get four +20% weapon skill enchants, they would add to +80%, and 75 + (80% * 75) = 180% * 75 = 135. Then, your actual damage would be measured as though you had a weapon skill of 135. As far as I can tell, skills scale linearly, so your final damage would be 180% * 50 = 90 (if this is true, you can just calculate +weapon skill enchants as though they are +weapon damage enchants).