One of the things that I use all of the time on Windows is the built-in calendar that appears when you click on the time in the Taskbar. Apple, for some reason, hasn’t implemented something similar for when you click on the menu bar clock, but a free little app called Day-O is able to do exactly that.
As you can see in the screenshot above this is app is about as basic as they come. The main purpose of using the app is the quick access it provides to a non-iCal integrated calendar. It makes answering questions like “when is the last Friday of the month” a lot easier, and is no doubt something that should be integrated into the OS.
In terms of settings there aren’t a lot of different configuration options available. You can adjust the format of the date/time that is displayed in the menu bar, and can choose whether or not you want the current date to display in the calendar-like icon.
This does not, however, formally replace the built-in Apple date/time display in the menu bar. That means if you wanted to you could continue using the Apple date/time, and in the preferences of Day-O you can erase all of the formatting options. That will result in only the icon being displayed, and clicking on that will still give you access to the calendar while the date/time display would remain the same as it always was. However, you can also disable the Apple date/time display in your Mac settings go to the Date & Time options, and uncheck the option to “Show the date and time in the menu bar” under the Clock section. That way you can use only Day-O for displaying the date/time, which is the route I decided to take myself.
Day-O Homepage (Mac only; Freeware)