Place a wide rubber band around the arm of your sewing machine, to the right of the needle. Make sure it is straight to align with your desired seam allowance. The wide rubber will help keep fabric from slipping to the right and keep your stitches straight.
A magnetic seam guide is the most popular tool for sewing straight lines. It magnetizes to the metal sewing machine plate. It can be moved quickly and easily and works on any sewing machine with a metal plate. The guide is tall enough to keep thicker fabrics in place while feeding too.
Peel off the back of a sticky note pad and press it along the right side of your needle to act as a seam guide. Peel off another sheet when it's time to move it for a different seam allowance. This method has the added bonus of a place to take notes!
Masking tape or blue painter's tape can be easily removed to account for different seam allowances when you need them. Peel off enough inches to reach across the width of your sewing machine's arm. Press it in place along the measurement you would like for your project. It works well as a guide, but does not provide a lip to hold against the fabric like some other items on this do.
A presser foot with a built in ruler adjusts to your desired seam allowance. Move the guide to follow the edge of your fabric, and you're all set. The foot is universal for low shank domestic sewing machines including Brother, Babylock, Singer, Janone, Juki, Kenmore, and lots more.
This inexpensive gadget is wonderful for piecing quilts and small sewing projects. It includes two reusable guides that adhere to each side your sewing machine's plate. Then your fabric can simply feed between the guides. The kit also includes a small ruler to ensure your measurements are precise.