Cuckoo is the famous little clock Cuckoo for your Mac ! Cuckoo lets you define the type of clock you want to appear (classic or modern) as well as its size. Regarding the sound, you can choice three sounds that you can adjust the power and the number of sounds ... such at a certain time you can mute the sound of the cuckoo, for exemple at night !
Note that Cuckoo is not a classic clock that stays on the screen permanently ! the cuckoo clock appears only at full hours (ex., 1:00, 2:00, etc.), and only for a duration (comparable to the clock speaking out loud the time once every hour] you have to define in the application preferences.
Cuckoo works in background and is controlled via the menu bar.
Finally, remember Cuckoo is only compatible with Lion !