WonderTape is what is shown in my first photo, it is a removable 1/4" masking tape. You can use it to mark seam lines while quilting or in the many ways we've been discussing in this thread.
You can get it on the notion/quilting notion walls of most quilt stores and chains. Knowing me I got it at Joann with a coupon. The one roll will last me the rest of my quilting life, if I used it to help place quilting lines I'd probably go through more than just a few inches on a ruler here and there.
There is also blue tape in tiny sizes at hobby shops and larger home stores.