Mac OS X only: Free application DateLine is an attractive linear calendar that lives on your desktop, sporting iCal integration and simple but useful customisations.
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There's not much to the application beyond that. The calendar lives anywhere you want it to (just drag and drop it to where it looks good), though we particularly like the way it looks across the bottom of your screen. You can adjust the display level so that it floats above all windows, as a normal window, or—most useful in our opinion—display at the icon level. You can also adjust its display colours and transparency. DateLine puts a small dot on the current day, and double-clicking any day will open that day in iCal. DateLine is a free, open-source application, Mac OS X only.