Temples: offer a specific bonus just for being under your control as well as a spell that costs resources (probably an equal amount of all resources for balance purposes) and effects a wide area around the temple as well as a smaller area around your hero.
Serpent Temple: greatly increases the virulence of all poison and disease; casts a spell that severely poisons enemies.
Infernal Temple: reduces resistance for all enemies and makes them more vulnerable to psych effects; casts a spell that causes chaos.
Celestial Temple: increases resistance for all allies and makes them more resistant to psych effects; casts a spell that causes awe.
Elemental Temple: doubles positive weather effects for allies and negates them for enemies, doubles negative effects for enemies and negates them for allies; casts a spell that changes the weather and summons an elemental (fine - fire, rain - water, night - earth, storm - air).
Orb of Seeing: Grants a small area of vision around all enemy heroes and a larger vision radius around enemies' highest level keeps.
Spire: Blasts bolts of lightning at enemies. Very long range, very high damage, and very slow rate of fire; can be garrisoned, garrisoned units provide more stats than usual and don't change tower projectile/damage type.
Fort (WBC2 Dwarf Armory): Nearby buildings cannot be converted. Casts Earthpower (does not effect itself).
Warrior's Guild (WBC 1-2 Arena): Increases combat and attack speed of melee units as well as attack damage and armor for your hero. Casts Battle Frenzy, sending nearby units berserk, getting them drunk, and giving a bonus to health regeneration to considerably slow berserk damage.
Ranger's Guild (WBC 2 Fletcher): Increases attack range and attack speed of ranged units as well as movement speed for your hero (melee heroes benefit from the attack speed but don't become ranged; heroes with a ranged attack via skills or magic get bonus range). Casts Silver Arrow, causing nearby melee units to gain a ranged piercing attack.
Mage's Guild (WBC 1 Library): Increases max mana and mana regeneration of spellcasters as well as spellcasting chance for your hero. Casts Greater Empower, Empowering the effects of all allied spells (+50% effect).
And just for fun: Pyramid: Doubles the rewards earned from all quests.