I have a new gas cooktop and the lowest setting on the smallest burner still produces afast rolling boil rather that a very low simmer. How can I reduce the heat? 
1. Simmer/Burner Plate  You can use a simmer plate or something to diffuse the heat. For example:   http://www.thekitchn.com/good-tool-burner-plate-for-low-107122 Google: Heat Diffuser   From the first link, a suggestion is to use a cast iron skillet as a heat diffuser. That should work in a pinch.  2. Adjust the Flame  Alternatively, some stoves allow you to adjust the flame level. I'm not a technician, so please do this at your own risk, read the manual, etc.... 