Caffeine is an essential app for me. Nearly a year after the release of retina displays, however, Caffeine still doesn’t have a retina menu bar icon. After searching for and trying several, my current favorite is JimmyGreen’s Custom Icon which maintains the original empty/full metaphor (to signal if the app is active). Simply download the icons and follow the simple installation instructions.
You will probably lose these icons if Caffeine updates, but the app hasn’t been touched in two years. Hopefully it will continue to work through the next version of Mac OS.