Magnetic Seam Guide
A handy little tool to ensure your stitching does not swerve and wiggle even when your attention momentarily lapses or your grip on the fabric slips. To install this little guy, just place it on your sewing machine's throat plate (the metal plate with the seam allowances engraved on to it). Line up the fence with your desired seam allowance width and start sewing!
The packaging warns not to use this magnetic seam guide on a computerised machine due to the risk of interfering with your machine's hard drive. This is a small risk (most hard drives will only by effected by extremely strong magnets) but one you should be aware of!