Functions.php but better

Themes never work without some tweaking, and it is tedious re-applying changes to functions.php whenever you change your theme. This Functionality plugin lets you create a portable functions.php that works with all themes.

Install it, then cut all the code out of your theme function.php that is not theme related. Paste it into the plugin created by Functionality. Use the plugin editor instead of the theme editor whenever you need to update your functions.

Simple and effective. Much better than plugins that give you an admin form to manage custom functions.

Update: 1 Year and 5 months later, I’m still using it every day, and it has worked consistently through all WordPress updates.

Update2: 5 Years and 8 months later, I’m back again. For a while, I followed Shea’s advice in the FAQ about using his Code Snippets plugin. Because I had lots of snippets in a multisite installation. But on my new sites, I’m using a hosting company which improves security with modsec. So Code Snippets and its rival Post Snippets both cause ‘403 Forbidden’ errors when you try to save a snippet. This plugin avoids that problem.