wClock for Mac OS X is a simple donationware menu-bar clock application. Of all the replacement clocks out there (and I've only tried a few), I like this one because it's small, and the only major feature it adds is the one I want: click on the clock to display a little calendar. I'm sure I'll want a featureful alarm clock eventually, but for now, this fits the bill.

Newbie Mac tip: To disable the regular clock, simply click on it and select Open Date & Time..., make sure the Clock tab is selected, then uncheck "Show the date and time." To re-enable the regular clock, go to System Preferences and select Date & Time from the System pane.

Newbie Mac tip: In Mac OS X 10.3 (Panther), to have an application start up automatically when you log in, go to System Preferences and select Accounts, then select your account, then select the Startup Items tab. You can add new applications to the list of available startup applications by dragging them in, or by clicking the + button.