Although not technically a Template Snippet, this tip saved the day numerous times.
CSS sprites are very usefull in reducing page load and fluid mouseover effects with background images. Creating sprites and the appropriate CSS is a time consuming process however. But Brian Grinstead made our life a lot easier. Visit his website at instantsprite.com and see for yourself. You can upload all your images and the tool automatically creates the sprite and the CSS for you. Very fast and usefull when theming your Drupal theme!