Where to begin...
-I use Warhammer miniature terrain, model train trees and bridges.
-I've crafted piles of treasure featuring gold coins, colored gems, and weapons.
-small wooden boxes and chests (dollar store variety)
-cheep and gaudy rings, bracelets, necklaces, and other jewelry for magic items (the more gaudy the better, usually found at the dollar store)
-Halloween costume weapons such as swords, shields, axes, wands, staves (recently added Thor's hammer to the collection)
-Maps galore - usually handcrafted, but I'll print them from the computer onto brown packing paper if I'm in a hurry
-Hollow display books with printed spell lists inside to represent found spell books
-Crafted scrolls stored in wooden tubes, held closed with leather straps
-Sound effects in the background, such as rain, waterfalls, urban markets, dripping water in caves, jungle sounds, night time sounds, etc. There are a lot of free sound effects to be found on the internet.
-Number stands (simple plastic stand with a number on it, which is placed beside opponents when there are hordes of enemies to keep track of)
Anytime I see something that could be used around the game table as a prop, I'll usually acquire it. The pay off is seeing my player's eyes go wide when they see a closed box and they want to know what's inside...
When I wonder if something is valuable as a prop, I always follow this idea:
Optimist: Glass is half full
Pessimist: Glass is half empty
Gamer: Glass has a liquid? I drink it, what happens?