Chaos Orb

Just some up-to-date info for those looking for it, since most comments are years old now:

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• Chaos Orbs are a Cataclysm crafting reagent, mainly used these days by tailors to make Dreamcloth (since the craft doesn't have a cooldown unlike the other Dreamcloth crafts).
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• Dreamcloth can be useful to make Cataclysm transmog items, not available by other sources. It can also be used to make leg 'enchants' (spellthreads) for heirloom items.
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• Chaos Orbs' main drop source are from the last bosses of Cataclysm heroic dungeons. Since the dungeon is heroic you will only be able to loot one orb, per dungeon, per day, however if you have several alts you can get multiple orbs that way. Any character 90+ can solo them, but higher level will avoid trash aggro.
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• The fastest dungeon is definitely The Lost City of the Tol'vir followed by Zul'Gurub and End Time. The Stonecore and The Vortex Pinnacle are also reasonably fast and have the additional bonus of dropping a mount if you're interested in that.
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• If you have several characters above level 90 and you're after a few of these orbs, my advice is to do The Lost City of the Tol'vir followed by The Vortex Pinnacle on each of your characters. The two dungeons are close together and easy to get to via the Uldum portal in SW/Org. The time taken to clear both dungeons is about 8 minutes. Just make sure the dungeons are on heroic :P
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Hope this helps :)

This is confirmed as the new Frozen Orb equivalent. Drops from Heroics and only drops once per Heroic instance.

>>>>> As of patch 4.3, these are now tradeable and no longer BoP <<<<<

In 4.3, Chaos Orbs are no longer soulbound! Everyone will be able to roll on new orbs that drop, and any existing chaos orbs will become tradeable.

-Kaivax (Blizzard Poster)

(Source)

These are incredibly effective against opponents who stack their mana and play their cards close together. Too bad they're typically banned in tournaments.