There are tons of uses for it: masking off areas you don't want to paint (obviously), marking lines on something you don't want to mark up (such as a floorboard you're about to cut and install), holding down stencils, graffiti and so on. But my favourite application of late is as a quilting guide (thanks, Soule Mama!). As a matter of fact, I'm using it on my first mixed media project – a quilted pillow cover made with one of the designs I stencilled at Austen's crafternoon in March.
For this pillow top, I want to echo the hexagonal border of my painted motif, so I lay strips of masking tape along the edge of each line. The intersections of the tape pieces are the pivot points. If you're machine-quilting like I am, just make sure you slow down a couple of stitches before the end so you don't overshoot. If it looks like I'm going to do that, I just shorten my stitch length and hand-turn the wheel so I have precise control. Don't worry about sewing over the tape in some places – it'll work out later.
When each line is done, I lay down new strips of tape along the sewn lines to keep the pattern moving outward. I like the width of regular masking tape for this project, but if you want lines that are closer together, you might want to try slimmer washi tape. If you want super-close stitching, some craft supply stores carry a product called Tiger Tape, which comes in a variety of quilt-friendly widths.
The tape pulls right off the fabric when your stitching is done. Just be careful when you reach a point where you've sewed over the tape. Pull slowly and gently on the shorter side to make the tape tear where it's been perforated by the needle. If any small bits get stuck under the stitching line, just tease them out with a pin. Ta-da! Straight quilted lines, no nasty pencil marks.
And just in case you're not a sewing kind of person, here are a few other cool (or silly) masking-tape-themed crafts.