Extensions are a way for users to automate common techniques in Inkscape. Many of them are created and shared by other users, and can be used to simplify the creation process for a lot of different tasks.
Here we are mainly going to restrict our list of useful extensions to those that would apply to designs for a laser cutter.
Installing an Extension
Normally an Inkscape extension will consist of two files, a *.py and a *.inx file. To install the extension into your version of Inkscape, you simply need to copy those two files into the "extensions" directory. Normally the extensions directory can be found at ".../Program Files/Inkscape/share/extensions/". Once you copy both of the files into the extensions directory, the next time you open Inkscape the extension will be available under the "Extensions" menu (note that sometimes new extensions may be grouped into another category).