Tap/Untap decks repeatedly tap permanents for useful abilities and then untap them to do it again. Mana creatures, artifacts, and permanents with powerful tap abilities can become engines when their normal once-per-turn limitation disappears.
Pemmin's Aura can repeatedly untap the right creature, while Seedborn Muse refreshes your permanents during every player's turn. Cards such as Aphetto Alchemist can create combos when paired with permanents that produce more resources than they cost to untap.
