Drupal's FieldAPI gives site builders a wide variety of ways to attach custom data to content types and other entities. Sometimes, though, you're adding metadata -- stuff that shouldn't be displayed directly when a piece of content is shown, but can affect the content's relationships with other posts, or its overall appearance. The easiest way to do this yourself is to hide the field in question, then sneak extra CSS classes or IDs onto the node itself based on the field's value. A new module, Field Formatter CSS Class, automates that process and makes turning simple fields into extra CSS classes a no-brainer.
As one might guess from its name, the module adds a new FieldAPI Formatter type -- "CSS Class." When selected, the field in question doesn't appear in the visible content of the node or entity in question. Instead, the field's value is turned into a CSS class and added to the entity's wrapper div. This is most useful when combined with a List field or a Text field with a small number of allowed values. When combined with some simple CSS rules in a custom theme (or added via a module like CSS Injector), this allows content editors to make easy choices that affect a node's visual appearance without hacking complex markup or embedded CSS rules into the content itself.
Field Formatter CSS Class is also smart enough to sanitize the CSS classes that it adds -- even if you allow editors to enter custom text in a field, dangerous HTML attributes and CSS tricks will be scrubbed out. If you need a quick and simple way for editors to pass information about a node or other entity to the theme, Field Formatter CSS Class is a solution worth checking out!