LaTeX2e's filecontents and filecontents* environments enable a LaTeX source file to generate external files as it's run through latex. However, there are two limitations of these environments: They refuse to overwrite existing files, and they can be used only before the \documentclass declaration. The filecontents package removes these limitations, letting you overwrite existing files and letting you use filecontents/filecontents* anywhere. The author is Scott Pakin.