Also known as Key Envelopes, these have become a standard in the paper arts world! Also useful for just about anything that fits inside for treasuring, identifying, or classifying. All are available in golden kraft, some in white, also.
Available in several sizes (in inches):
Mini - 1-11/16 x 2-3/4
Small - 2-3/16 x 3-1/2
Medium - 3-1/8 x 5-1/2
Large - 3-1/2 x 6-1/2