The letter deck
Great for prototyping
I made this deck because I wanted a simple set of cards with letters on them for prototyping word games. The deck is simple: 100 cards, at least one card for every letter, along with extras of most letters (roughly following letter frequency in English).
Here's an example of a quick game I made with these cards, called LUCKY: http://filthyditty.posterous.com/lucky-a-letter-deck-game-prototype