Of the stuff on your gaming table, of course.
One of the problems with adding more bits to the gaming table, is that the table is already pretty busy to begin with. Miniatures. Dice. Rulebooks. Pencils and paper. Booze. Pizza. Character sheets. Terrain and scenery. Maps. Dungeon Masters. For good or ill, all of them end up demanding space at the table.
This means that adding resource cards to the game is just another thing competing for your attention. Since the whole point of adding cards is to simplify resource tracking, it seems counterproductive to do so without giving this more thought.
I had originally thought to use the typical 2 1/2 by 3 1/2 inch cards as resources. But lets say you end up with 20 items in front of you. Laid side by side, end to end, they take up 175 sq inches. A typical A4 or 8 1/2 by 11 character sheet takes half of that, 95 sq inches. So cards used in this fashion are going to end up using a great deal more space at the table.
Having said that, while most of the available resource cards (such as the Dungeoneer or GameMastery ones) have attractive art, they also have the name of the resource at the top of the card, so it's a simple matter of having only the name of the item showing, rather than the entire card, including art.
Flash back to Magic Realm or Civilization. Both of these games use smaller resource cards, say 1" x 1.25". For most items, we really don't need the level of art that you might see on a Magic: The Gathering card. All we need is the item name, a small picture of the item, and perhaps a couple of words at the bottom describing the related mechanic, or other game-related information.
Unfortunately, I could not find any cards, that small, that were suitable for my purposes. This meant a home brew solution.