Powerbalance 
Enchantment
$0.32 #2443 0.41 2.2 years
Whenever an opponent casts a spell, you may reveal the top card of your library. If you do, you may cast that card without paying its mana cost if the two spells have the same mana value.
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Cards similar to Powerbalance expected a card that isn't here?
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Rashmi, Eternities Crafter85%
$0.39↑#3551↓0.25↓9.9 years↑
"Whenever you cast your first spell each turn, reveal the top card of your library. You may cast it without paying its mana cost if it's a spell with lesser mana value. If you don't cast it, put it into your hand."
Galvanoth78%
$0.31↓#13054↓0.20↓15.6 years↑
"At the beginning of your upkeep, you may look at the top card of your library. You may cast it without paying its mana cost if it's an instant or sorcery spell."
Yennett, Cryptic Sovereign78%
$0.37↑#11069↓0.27↓8.1 years↑
"Whenever this card attacks, reveal the top card of your library. You may cast it without paying its mana cost if its mana value is odd. If you don't cast it, draw a card."
Mind's Dilation77%
$0.40↑#26560.76↑10.1 years↑
"Whenever an opponent casts their first spell each turn, that player exiles the top card of their library. If it's a nonland card, you may cast it without paying its mana cost."
Descendants' Path76%
$0.91↑#21820.3314.3 years↑
"At the beginning of your upkeep, reveal the top card of your library. If it's a creature card that shares a creature type with a creature you control, you may cast it without paying its mana cost. If you don't cast it, put it on the bottom of your library."
One with the Multiverse75%
$2.65↑#1919↑0.74↑3.8 years↑
"Once during each of your turns, you may cast a spell from your hand or the top of your library without paying its mana cost."
Sunbird's Invocation72%
$0.25↓#1885↑0.328.9 years↑
"Whenever you cast a spell from your hand, reveal the top X cards of your library, where X is that spell's mana value. You may cast a spell with mana value X or less from among cards revealed this way without paying its mana cost. Put the rest on the bottom of your library in a random order."
Dragonlair Spider70%
$0.47↑#8231↓0.10↓14.3 years↑
"Whenever an opponent casts a spell, create a 1/1 green Insect creature token."