I love simple utilities, and Dateline certainly fits that bill. This small application puts a linear calendar on your screen that shows a dot on the current date, and will jump to a given date in iCal if you double-click it. That's pretty much the full extent of what Dateline does, but in this case simplicity equals elegance.
Considering its functionality, Dateline contains a reasonably complete set of preferences that control how it is displayed. You can choose the colors and opacity for each visual element, the size of the overall date line, the window level (as Gruber points out, setting this to Desktop icons seems to be the most reasonable setting), whether or not to show the month's name, and the option to hide the app's icon in the dock.
After trying it for only a couple minutes, Dateline immediately went into my list of applications that automatically open at login.