Thread Drops with Ring by Access Commodities
I've used these for years -- they are very nice for storing your threads. The white cards are smooth and sturdy (they are paper, so you can write on them.) And the rounded shape means your threads won't get snagged or pulled. A tip from me: if you want a temporary way to mark a thread drop, use the top (sticky) portion of a Post-It note. You can cut a little piece of that sticky paper, write your number or symbol on it, then peel it off when you're done and ready for a different color.