These are some of the cards that I think are underused, despite their usefulness.

• Time Tide
• Idols
• Sculpture

Time Tide

This card is pure magic. Any deck that relies on stat modification is screwed. It heals up all of your creatures, even if they're Phantasmed, for the same cost as Refresh. If you're having nightmares about an invincible army of Powder Eaters rolling over you, this card will save you. Consider this one for a 2-slot in almost any deck. Unless, of course, your deck relies on stat modification, in which case you'll be hoping someone else isn't packing it.

Idols

Many decks are completely hosed by the presence of one of a few idols. Marble and Ebony are both debilitating to a number of different decks. Granite makes a number of cards in your opponent's deck essentially dead draws. Ivory and Lead both can be huge nuisances. I like to pair idols with Mass Phantasm. This keeps them from being destroyed by direct damage, which means they have to picked off one at a time. I also like to use two 4-slots of different idols in the same deck. This makes it much more likely that you'll get at least one of them to stick for a prolonged period of time.

Sculpture

Nearly everyone forgets about this card. It rolls through idols. It transforms everything that can't be targeted by spells or effects except for Gearion. Nearly all Support creatures get stoned. A lot of the more annoying creatures are wiped. The ONLY thing that stops Sculpture is HP > 30. Combine with Plague, creatures with 40+ ST/40+ HP, and Acid Rain or Holy Blight for mass destruction.