Cuckoo for Mac

From toastycode:

Cuckoo brings clock chimes back to the Mac. Play a beep on the hour---or on the half-hour, or every five minutes if you must. You can use one of Cuckoo's tasteful built-in tones, or any of your Mac's installed sounds. Cuckoo has its own volume control, so your chimes can be a subtle part of your work environment. And Cuckoo will go silent altogether if you want (for example, while iTunes is playing music or videos, or when the screen saver is active).