Game files contain a bit of unused content.

Restoration of Erathia

Unused creatures animations and sounds

Centaurs have unused shooting animation and unused shooting sound (HoMM2 leftover):

Alternative Beholder's death animation and death sound (realized in Horn of the Abyss as a result of death from a spell):

Alternative Gold Golem death animation (realized in Horn of the Abyss as a result of death from a spell):

Spell casting animations for:
Sounds of a spellcasting for:
Alternative liches melee sound (HoMM2 placeholder)

Alternative spell effects and sounds

The game contains alternative animations of Berserk, Bless, Curse, Stone Skin, Cure, Dispel, Forgetfulness, Frost Ring. They're very similar to the final animations, just slightly slower coming up and fading out.
Alternative sounds for Slayer, Fireball, Haste, Magic Arrow and Fire Shield.
Alternative Fireball animation:

Unused petrify animation probably for Medusas and Basilisks:

Adventure Map Objects

Certain maps include Market of Time (e.g., Titan's Winter, Warmongers in RoE 1.0).
Alternative Windmill
Alternative Gnoll Hut (present in Gelea's Champions scenario)
Alternative School of Magic
Alternative Campfire
Red Obelisk for underground terrain (realized in Horn of the Abyss)
Alternative underground hole (in-game graphic is only a placeholder)
Adventure map versions of Ballista, Ammo cart, First Aid Tent, Spell book, Grail and Catapult, however only with placeholder graphics

You can still easily add these Adventure Map objects into your map editor. Map with this objects will work correctly even for players without these objects enabled. For SoD simply put this Objects.txt in your "data" directory or insert it in H3Bitmap.lod using Grayface's MMArchive. For HotA put this EdObjts.txt in your "data" directory or insert it in HotA.lod using Grayface's MMArchive. All unused objects can be found under "Towns" category.