This is a compendium of formulas. I'll try to keep this thread updated. For now it's just the portion of effective life.
Effective life: On average, the amount of raw damage you can sustain before you die. E.g. If you have 10000 life and 50% damage reduction, you have an "effective life" of 20000. This is useful when selecting which item is better. "Protection" in item selection tool tip doesn't take resistances to account for example.
Effective life: Total life / Total damage reduction.
Life:
(276+Vitality*35)*(1+Max life modifiers)
Total damage reduction:
1-Dodge chance *
1-Damage reduction from armor *
1-Damage reduction from resistances *
1-Damage reduction from being a melee character (30%)
1-Other damage reduction.
Blocking works differently. It has a chance (Block percent) to absorb a fixed amount of damage (Block amount). The damage is calculated after resistance and armor reduction has been applied.
As far as I know all damage can be dodged, resisted and blocked. I.e if you are poisoned or standing in a fire, you can dodge that damage.
Damage reduction from armor
Armor / (50*Monster level + Armor)
Damage reduction from resistances
Resist / (5*Monster level + Resist.
Dodge chance:
Dodge chance from dexterity * Additional dodge chance modifiers multiplied./code]
The dodge modifiers should be should be multiplied instead of added.
The dodge chance increase from each point in dexterity works like this:
[code]Dex Range Dodge% / Dex
0 - 100 0.1
100 - 500 0.025
500 - 1000 0.02
1000 - 8000 0.01
8000 Dexterity = 100% chance to dodge
Armor:
(Strength+Armor from items)*Armor modifiers
Resistance:
(Intelligence/10+Resistance from items)*Resistance modifiers
Note that each point in Dexterity, Intelligence and Strength increases effective life by an equal amount (approximately). Also note that there is no diminishing returns in terms of effective life on neither armor, resistance or dodge chance.
Monster Levels
Act1: 61
Act2: 62
Act3: 63
Act4: 63
Which stats increases my effective life more?
Let's say you have 40000 life, 5000 armor and 500 resist all. These modifiers should be equal in value and each represent a 1% increase in effective life vs monsters in act 3&4 inferno:81.5 Armor (or 81.5 Strength)
8.15 Resist all (or 81.5 Intelligence)
11.5 Vitality You generally get more effective life if you try to keep Armor, Resistances and Vitality balanced rather than going to the extreme on one thing. One exception is if you use skills that increases these by a percentage.
Items generally have better modifiers for vitality to increase your survivability, but keep in mind that health potions, health globes, +life on hit items and heals doesn't scale as it does with damage mitigation. Resistance bonuses on items seems to be much better than armor modifiers, so it's generally better to get resist all modifiers as a second choice.
Disclaimer: It's not complete by any means, and also not proof read 
Additional links/Sources:
http://www.armadagaming.com/showthread.php?459-Armor-Resistances-and-Effective-Health
http://www.clicktoloot.com/p/combat.html | 
