The Tucker Trimmer 1, designed by Deb Tucker, has many uses, and I find it to be one of the most versatile rulers out there. It allows you to make Half Square Triangles, Quarter Square Triangles, And Triple Square Triangles larger than needed, and trim them down to perfection. My favorite use for this ruler is to make a Cat's Cradle (aka Shaded Four Patch) unit larger and trim it down. To my knowledge, the Tucker Trimmer 1 is the only ruler that works for this unit, making it invaluable to me.
This ruler trims the above units in whole and half sizes from 1" to 6". It comes with a booklet, and there is a wonderful video you can watch as well.
It's a must have for all quilters!