Whether you have made your own sets of vocabulary cards or another ALT (or Japanese teacher) has, generally a lot of effort goes into making them. If you want to put them to use, here are a few different ideas of how to use those big sets of English vocab cards.
Games using English vocabulary cards
Karuta
The students get into pairs or small groups, spread the cards out between them and try to be the first to slap their hand down on the right card as you call out each word.
See the karuta page for more details.
Relay gesture game
Students make groups and one pair from each group comes up to their own pile of cards. One student takes a card and gestures; the other guesses the gesture. How it goes from there depends on the variation of the game.
See the relay gesture game page for variations.
Memory match card game
Each group of students gets two sets of cards (ie. pairs of each card). All cards are laid face down and one by one, students flip two cards and try to make pairs.
See the memory match card game page for the full description