This concept of anti-magic items can also be fun as well. I had a few thoughts about this, and this is what I came up with:

Black Amber
A glossy, dark material, which can be either solid or liquid which attracts all magical spells in its vicinity. If a spell happens to hit it, then the spell is then contained inside of it until broken. From this point onward, it will not attract any more spells. If the person who threw the spell manages to get the Black Amber, they are able to re-absorb the spell, allowing it to be used again. Re-absorption will destroy the amber.

Although I mainly thought of using it for defensive purposes, it can be used for Wizards and other spell-using classes to store additional spells.

Mage-Glass
Transparent material which seems to defy the laws of gravity, floating in mid-air. It is attracted to any magical source. Casting a spell nearby the glass will cause the glass to throw itself toward the caster, dealing damage depending on the size of the glass (Small shards doing 1d4, larger ones doing 1d6, and so on). They typically come in shards, but can also be made into marbles to deal bludgeoning damage, or fashioned into bottles to hold acid (or if used in juncture with the Black Amber above, a fireball spell ). It loses its attraction to magic (and its utter defiance of gravity) once it breaks.

This is a lot more trap-like. Any spell, even simple ones like detect-magic, attract this stuff. Gotta keep them wizards on their toes.