Clearcasting is both a term used for effects which cause the next spell to be free of cost and more specifically the name of buffs which provide this effect.

• Buffs named Clearcasting
  • Arcane Concentration's effect (Mage talent)

• Buffs providing a clearcasting effect
  • Inner Focus (Priest talent)
  • Omen of Clarity (Druid talent)
• Similar buffs
  • Elemental Focus (Shaman talent) used to be a clearcasting effect and still has a similar mechanic
  • Lock and Load (Hunter talent) allows you to use two Explosive Shots or Arcane Shots without Cooldown.

When Clearcasting activates casters usually use a more mana intensive spell next to fully exploit the benefit of the talent. Also, casting spells when a clearcasting effect is active doesn't reset the mana regen timer.