Spike's scarf...

Actually it's own being of unknown origin, it grants Spike (and Anki) various abilities, the largest of which is the ability to speak coherently to humans and other beings..well.. english at least. This is even despite that it can't speak or even use telepathy.

It's been known to grant the ability to use a few forms of magic in the forms of fireballs, have an ice walk on water, and tiny bit of size change, though these are rarely available.