Metronomo is a menu bar application that simulates the sound of a metronome. Beats per measure, tempo, and volume can each be adjusted, and the application can be activated and deactivated with a customizable keyboard shortcut. There's also the option to open the application at startup.
Rather than attempt to replicate the sound of an analog metronome, Metronomo uses short beeps to mark beats, with a higher-pitched beep marking the upbeat. The corresponding sound files can be replaced by accessing the application's package contents, but are not otherwise mutable. Available tempos range from 20 to 240 beats per minute, and available beats per measure (i.e. the upper number on a time signature) range from 1 to 8. (There's no explicit option to turn off the differentiating upbeat sound, but the same can effectively be achieved by setting beats per measure to 1.)
The interface is smooth as silk and integrates nicely into the Mac interface. This app should come in handy for analog and digital musicians alike, albeit perhaps more so for the former.