Battery Guard is a battery charge and health monitor that resides exclusively in the menu bar. It provides information about the battery's current charge, current maximum charge, original maximum charge, temperature, and age in cycles and months. It also indicates the amount of time remaining for running the device off of the battery if it is not currently charging.
Most of the extra features of Battery Guard are there to promote its developer, Avocado Hills, despite the fact that there's no mention of the app on the developer's website. There is a launch at startup option, which is useful for those who don't know how to enable this through System Preferences yet always want to track their battery health (perhaps a small intersection, but useful nevertheless). There's also an option to display the time or percentage of battery left, thereby making the application a completely viable alternative to the built-in customizable battery menu icon.
Sam's Protip: My one dismal experience with a third-party Macbook battery convinced me that it's worth the extra dough for an official Apple-made one. If you're replacing your battery, do recycle the old one!