- Fotodiox Graduated Gradual ND
- Tiffen 67mm Color Graduated Neutral Density 0.6 Filter
- Cokin Graduated Neutral Grey G2-SOFT ND8 0.9 Filter
They're called grads by experienced photographers and they can be lifesavers in tough situations where not even the awesome capabilities of today's cameras are enough.
Our eyes are use to adjusting to powerful contrasts and to making the necessary changes in order to see light correctly. It's like having various ISO values in the same frame. Cameras obviously can't do that and that's why they need a little help.
A lot of photographers favor post processing for light adjustments and that's probably how HDR was born. Here's the thing. Graduated ND filters give you better results and they do it instantly. If you're serious enough about photography and have gotten over the HDR-is-so-cool phase, you'll appreciate the natural feel of a photo taken with the right filter.
But getting a little back to basics, a grad is essentially a clear piece of plastic or glass that's grey on top. They're probably most used by landscape photographers because they're the ones who deal with harsh light and powerful contrasts a lot. Grads help preserve details in shadows and keep the colors in the sky.
They come in a lot of variants and that's probably what scares a lot of less experienced photographers. Some of the mare coated others not so much. A coated ND will work better when there are reflections in front of the camera.
Grads are either typical screw-in filters or rectangular sheets that need special holders. The later type seemed to be favored by a lot of landscape and nature pros for some strange reason. They also take longer to mount and if you're not careful, that nice light could go away before you take the shot.
If you're a firs time ND user, I recommend trying a screw mount filter as it will feel familiar and easier to use.
Here's a video from Adorama TV with photographer Corey Rich telling a great story about how he uses graduated ND filters.