A staple of many costume designers, being extremely simple to sew, cheap, very flattering to wear, and infinitely adaptable to many figures. Also known as an "angel gown", it consists of a simple oblong of fabric, at least as wide across as the hip measurement, plus ease, about twice as long as the measurement from shoulder to ground, folded in approximate halves.
A round neckline is cut, and interfaced. The front half is caught at the waistline (safety pins can be used, if this is a one-time-only affair) and sewn down to about the back of the knees. (Additional material might be inserted here for someone of, say, Orson Welles proportions.) The rest is hemmed for a walking slit, with the bottom hemmed for a regular skirt. The top part hangs free, can be a bit longer (if a train is needed) and is simply hemmed at the bottom.
Now then. Since the whole outfit hangs free from shoulder to the floor, we have quite the duster effect here, as detailed by J. Peterman, that is, if you aren't tall and skinny, you will instantly become so. (Think Young Guns here.) You will also get a lot wider in the shoulder, if you are not, to the point where you might want a pair of brooches to gather up the material if you are female and trying to look pretty. If you are male, well, then, no one will think you less than masculine, if only because you will look powerful. This is a Good Thing if one is, say, the Angel Michael, appearing at the end of Millenium Approaches, or simply wanting to be a kick-ass fightin' angel, as opposed to a fluffy girly angel. (Somehow I remember a version of this on Star Trek:TOS, as well. Maybe the Vulcans?) Also, my Scots representative tells me that, having little front ease, nor a sporran, you guys ought to keep your emotions in check, lest everyone know how glad you are to be there.
It will also give you a cape to play with. Dancingwise, this is very good. If the outfit is made of say, unbleached muslin, the cape will float behind you if you walk tall and just the right speed. (More sedately, a length of light chain might be sewn into the cape hem for a trailing train, which, under the right conditions, can also be made to billow.) If you catch up the outside edges lightly, and pirouette, the cape will spin horizontally, or veronica. Eveningwearwise, expect the waiter to want to take your coat, unless you tuck it in. At which point, you can shrug, and indicate that you are all of one piece, and arch your brow, and say "Do you really want to?"