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Although not technically a dedicated portrait plugin, Nik Software's Color Efex Pro has all the tools you need to improve your portrait shots, and it is used by hundreds of pros for just that reason. Essentially it is a collection of filters that you can apply to your images, but there are two features in particular that make it appealing to portrait photographers.
The first is the Dynamic Skin Softener. This allows you to select small, medium or large details on skin and soften them for a smoother, more diffused look.
The second big plus for portraits is the U-Point technology. They work in a similar way to Control Points on Nikon's CaptureNX2, in as far as they let you make selective adjustments to specific areas of your images without masking or selection tools, and they work very well. Other features like the White Neutralizer or the Color Cast tool also have uses with processing portraits, while all adjustments are made as a non-destructive layer mask. So, it might not be a portrait plugin per se, but it is still one of the best portrait plugins for Photoshop.