(Apologies in advance- I'm going from memory, so I may be talking about cards you won't be able to use.)
Decking an opponent has many paths.
Besides Millstone, there are several other artifacts that essentially do the same thing or a variant upon it (I don't have my deck on hand, or I'd name them off). One mills a # of cards based on how much mana you pump into it, but if it mills a creature, it stops milling and puts the critter in play under your control. As mentioned above, Jester's Caps are potential deck-wreckers.
One classic combo I saw in tournaments was Braingeyser combo-ed with ways to boost mana- Mana Flare, usually. However, everyone has their faves- Urza Lands, Tolarian Academy, Elves + Priest of Titania + Gaia's Cradle (or whatever land it was), and so forth. Use Blue cards & Howling Mines to draw your Braingeyser, then tap for a billion mana and poof! No more deck!
Other cards that work well in this include some Black cards- Hypnotic Specter, you'll recall, makes your opponent discard from his hand- a real pain when your deck is getting milled besides. Black also has a host of other discard cards.
A less common approach, but one I've used, was a Red combo of Shocker + Fire Whip. Shocker is a 1/1 Red critter that makes you discard your hand and draw a new one if it damages you, and Fire Whip is a Red creature enchantment that lets you tap the creature to do 1 damage to any target. This means you both mill and cause discards at the same time- up to 8 cards in normal circumstances, more if your opponent is drawing extra cards for some reason.