Beer bottle caps.
I'm sure this, or something to this effect has been posted before but......
I was cleaning up yesterday after having a few friends over saturday night and when I was putting away the beer empties I also had a few hand fulls of beer caps, just as I was about to pitch them in the garbage it dawned on me, rinse off the caps and put them in a ziplock bag. I now have 20 or 30 disposable cups for mixing small amounts of paint in, or for putting a few drops of glue/crystal clear in while building.