When you are just getting started with WordPress, you may not be ready to write your own code. After a while, you will most likely end up finding or coding custom functions to improve your website. Plugins such as My Custom Functions allow you to add your own functions in a safe fashion.
My Custom Functions does not require you to directly edit your functions.php file. Just add your code in the appropriate field, and the plugin does the rest for you. It keeps you from breaking your website with typos and other small mistakes. My Custom Functions offers syntax highlighting and line numbering to make your life easier.
My Custom Functions is useful for those of you who are somewhat familiar with PHP but would like to add custom code to your site without breaking it. This is a free plugin.