Power-ups appear on the game field once you match four or more pieces. These are what's available in the game right now:
Firecrackers explode all neighboring tiles. You can get them by matching four pieces of the same color.
Dynamite explodes everything within a three-tile radius. You can get it by matching six pieces of the same color.
TNT explodes everything within a four-tile radius. You can get it by matching seven or more pieces.