Items dropped in the ground, like weapons and shields (even by NPCs); Ghostly Remains, Ice Piles, and Wisp Cores are currently not cleaned up at all (they remain indefinitely, contributing to save game bloating and game lag).
In the PC, this is fixed by the Unofficial Skyrim Patch (mirror link):
Ghostly Remains, Ice Piles and Wisp Cores will no longer permanently remain and bloat the save file (they will either disappear on exit of the cell or when the cell resets).
NPCs will no longer drop weapons or shields as separate objects, as these are never cleaned up which greatly adds to save bloating and litters the gameworld (the items will still appear to be dropped, but will access the corpse's inventory to be taken and be cleaned up with the corpse if left)
Ash Piles used to remain indefinitely (was not properly cleaned up by the game), but has been fixed since official patch version 1.4. Nirnroot lighting or glow objects also used to be not properly cleaned up by the game, but has been fixed since official patch version 1.6. Related official patch change logs: