Nesting is a process of arranging shapes, especially inside of each other, to minimize the total area used. Nesting software is very useful for CNC applications since it allows the user to minimize their material costs. There are many nesting applications available at vary price points and with various features. SVG Nest (http://www.svgnest.com) is a free and open source, web-based nesting application.
Edit: There is now a new option from the creator of SVG Nest called Deep Nest. It is still open source but is "Pay What You Want" to support the developer. You can find it at https://deepnest.io/