The built-in clock of Mac OS X is pretty cool, but it's a bit obscure for some folks. If that includes you, try DeepClock, an US$0.99 app available at the Mac App Store.
It's bigger than the default OS X clock, which can help keep you focused when time is of the essence. DeepClock can be set to launch when you boot up your Mac. You can keep the it in standard, floating view or have the utility sit atop all applications (even in full screen mode).
One of DeepClock cool functions is "Events," which allows to add small reminders that can be displayed on your desktop to remind you of important events. The app is extremely customizable. It sports 15 styles of clock face and eight calendar styles in an appealing interface. You can also set the app to vanish when the cursor goes over it so it doesn't block what's underneath.
If you're totally happy with the OS X default clock, you can pass on DeepClock. However, if you're looking for a replacement, this is a functional, attractive, and inexpensive app.