I rarely keep yarn labels after I finish using the yarn they describe. Sometimes that leads to problems. Recently, I needed to wash a sweater, but I just could not remember whether the yarn was machine washable. Since I did remember the name and brand of the yarn, I Googled it. That's when I found Yarndex.
Yarndex is an online database of yarns, including discontinued lines, that allows you to search by name, brand, fiber content, color, or texture. When you find the yarn you are looking for, it gives you the label information, and, if available, a photograph of the yarn. Truly a wonderful resource, and well worth keeping bookmarked!