This is great. Not many utilities use the corner spaces of the menu bar (to the left of the Apple menu and right of Spotlight) and I like how BatteryViewer does it.
There are a lot of configurable features not currently mentioned in the summary so I'll include theme here:
BatteryViewer is a fun Preference Pane and background application pair which displays the level of your computer's battery. Generally, it does this by displaying a bar graph (there are other ways that you will discover).
The size and location of the graph can be selected. The colour (and transparency) of both when the battery is empty and when it is full can be specified. Battery levels between empty and full will blend the two colours and their transparencies.
There is also an option for showing the percentage full as well as the time remaining (until empty or completed charging).