Ability score bonuses (strength dexterity, etc) won't stack across multiple items.
However skills (hide, pick lock, etc) WILL stack. So wearing three items that boost the hide skill by +5 each will give you +15 to that skill.
Bonus hit points granted by items also stack, as do percentage immunity to energy types. Two items that grant you 50% immunity to fire would result in 100% immunity when both are worn. However if you exceed 100% immunity you actually start to become vulnerable instead (150% 'immunity' would result in 50% vulnerability, taking 1.5x the usual damage).
Damage reduction (5/fire, 10/cold, etc) won't stack though.
Most armour class bonuses won't stack when granted from multiple items, except for dodge bonuses. However I'm not aware of any items other than boots that grant dodge bonuses, and you can't wear two sets of boots at once. 