I have one called "Menu Calendar" it puts an icon in the menu bar and when you click on it a small calendar pops down from the menu bar.
But it's just a calendar, it doesn't show appointments or other stuff. You can go from month to month and years.
That "Calendar" app is great, that CubaMark suggests below.
I have two Calendar applications which I use frequently...
Jordan and I both use MenuCalendar, a great little utility which resides up beside the clock / battery icons on your menu bar. Very useful for quick access.
Another which I think is a great implementation is "Calendar" http://sailesha.com/Calendar which has some excellent features. Command-click on a day to add an appointment, set background & foreground transparency, force it to always be "under" other application windows, and a wickedly cool feature: if it has an internet connection available, it automatically checks the nearest weather office and displays a four-day forecast with icons for clouds, rain, sun, snow... very nice added touch.