One of the most useful (and best crafted) filters for portrait photographers is the Carl Zeiss Softar II. Unlike many other softening filters, it's not too heavy-handed, yet it adds a flattering quality to both subject and background.
I've had one on hand, but when I was researching their availability, I discovered that B&H Photo has the 55mm Softar II in stock for only $39.95 ($59 off the regular price). Film shooters who love portrait photography, take note!
For this image, I used a wide aperture and placed the Softar II over the front of the Zeiss 50mm. No retouching was required. And the image has a pleasing quality that doesn't feel too heavy-handed.
Along with a polarizer, this is a filter that I always have in my bag of analog gear.