The menubar on your Mac is black text on a white background, and even if you tell OS X's system preferences to set the menubar to translucent, it's still a little on the bright side. If you're using dark wallpapers or prefer your monitor not glow quite so much, MenuBarFilter will switch it around so it's white text on an all-black background.
MenuBarFilter is completely free and open source, and the code is actually largely taken from the previously mentioned screen-darkening tool Noctune, which you may have seen in some of our Mac-centric featured desktops.
Unlike Nocturne, which will swap your entire screen (or elements of it, if you choose) from light to dark, MenuBarFilter just handles the menubar. Any menubar icons will switch to white or grey so they stand out against the black background. Plus, even though Nocturne works with newer versions of OS X, it hasn't been updated in a long time, so MenuBarFilter can get the job done in Snow Leopard or Lion (and most likely, Mountain Lion as well.) Give it a shot and let us know what you think.