I have trying out several thirdparty Photoshop plugins as part of my imaging workflow. The first is a plugin from Alien Skin that attempts to emulate the attributes of classic film like Velvia. The plugin is suprisingly flexible. The settings out of the box are a bit too strong for my liking but I have found myself going back to this software for enhancing certain landscapes. I also like the b/w conversion.
Nik Software produces a highly regarded sharpening plugin. I downloaded the demo version and I could not get it to work in Photoshop. And, at over $300 USD, I think I would like to at least assess its performance before I buy it. I hate it when demos do not work.
Pixel Genius, on the other hand, has a sharpener tool with a fully functional demo. A multi-stage sharpener with a great deal of user control, I really like its impact on my images. The site also offers a couple of downloads that describe workflows for digital photographers. Some good insight can be found in these examples here.