While working a sweater in Fair Isle or Stranded Knitting, one often has 15-20 balls of yarn to keep straight. Not only do you have to have the colors correctly identified for the different bands, but you also have to prevent tangling. Some wonderful blogger had this tool but I just can't find her blog's link. I had it on my 'wanted' list for about a year and a half and finally indulged. It is better than I could have ever imagined.
The original purpose is to store cups or other small items. It folds completely flat and I could have opened the last section for more yarn, but chose to unsnap it for my color card and notes.
Once closed, I can store it on the side or even upside down and nothing is disturbed when I reopen the case. I can't express my joy at this wonderful tool.
It is called a Canvas Storage Organizer and costs under $20.