BatteryWorks is a fun app which displays the level of your computer's battery. It does this by displaying a graph/thermometer or colouring an area of your screen a particular colour to show you the battery level. These can be on your menubar or desktop.
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.