Oracle Text

Activated abilities of creatures cost 1 less to activate. This effect can't reduce the amount of mana an ability costs to activate to less than one mana.

Card Rulings

10/4/2004 The cost reduction can be applied to additional costs.
10/4/2004 It will not add a to abilities with no generic mana in their activation cost.
10/4/2004 It will never reduce any colored mana portion of an activation cost.
7/15/2006 Can’t reduce Snow mana costs.
6/8/2016 Activated abilities contain a colon. They’re generally written “[Cost]: [Effect].” Some keywords are activated abilities and will have colons in their reminder text.