How do I convert a PDF to Kindle and Nook format?

Query by jonrwarren: How do I convert a PDF to Kindle and Nook format? Can someone stage me in the right route, ideally to a software package program that converts a PDF e-book to Kindle E-book format. Greatest solution:
Solution by Myriam
Calibre. Really easy and valuable.
Know far better? Leave your personal response in the comments!