One of the most popular Sewing Machine attachments is, without doubt, the Even Feed Foot.  Sometimes referred to as the "Walking Foot" this clever attachment is designed with a set of built-in feed dogs (teeth) that grip and advance the upper layer of fabric whilst working in unison with the sewing machine feed dogs as they grip the underside of the fabric.  This action helps to prevent layers of fabric from "shifting apart" as they move through the machine.
Use this foot when matching stripes and checks, sewing draylons and velvets, making up knitwear or when sewing long seams on curtains especially if matching patterns.  It is also useful when tackling leather, vinyl, or plastics and other difficult to handle materials.
It is an absolute must for quilters.  Used with the Quilting guide it helps to maintain perfect parallel lines when stitching and keeps the fabric and batting layers from feeding through unevenly.