Nd filters, Glass.

It depends on what you really want. The simplest ND is a screw on filter, which most likely will suffice for the majority of photography. Variable ND filters are a bit of a mixed bag and, nowadays, with HDR capability in software, may not be all that necessary. The main thing to be aware of is that not all ND filters are made to exacting standards and this is one area where price is often a determining factor. If you want possibly the best in ND, then B&W filters are a clear choice. But whatever you do, don't buy from the bargain bin.

 

It depends on what systems you have. If have only m4/3 and are planning on a wide angle. I'd highly suggest Cokin P, and using singh ray and a couple specific cokin filters. The singh ray ND's aren't too expensive, and they really do cause the least amount image degradation. If you don't plan on the ultra wides, specifically under 12mm might as well get the cokin A size and use only singh rays, as the A sizes are really affordable at that size.

 

http://www.singh-ray.com/
They're a smaller company that makes high end filters. Known for the polarizers, and ND's that have no color shift or very specific color shifts. Frankly, you won't find a better polarizer. And anyone using m4/3 and doing landscape work should be using them as that is literally one of the best features of the system, Singh ray filters are affordable at smaller sizes. The cokin holders I specifically suggest because singh ray has sprocketed CPL's that insert into them and work great. Lee holders are kind of scary to be putting your shiny new expensive singh ray CPL in.

 

http://www.leefilters.com/index.php/camera/system-seven5

 

I just got a set of Hitech ND grads from amazon and took them on my trip. They work pretty well. Cokin P are much bigger. The hitech's fit in the Cokin A filter (you can get one for $5 on ebay). These are much smaller and fit in a small pocket and are meant for M43 sized camera lenses. They are on sale now for less than $40 for a 3 ND Grad kit. No color cast.

http://www.amazon.com/Formatt-Hitech-67x85mm-2-67x3-35-Neutral-Density/dp/B00A3MGVIK/ref=sr_1_11?ie=UTF8&qid=1410528358&sr=8-11&keywords=hitech+85mm+filter+kit

I got the hard edge, because I read they are better for telephoto....but they have soft edge kits too...as well as other things like sunset.

I got a kit with sunset, blue filter and twilight by accident and got a refund and kept the filters. They have some usefulness too.

I learned about HItech from other posters on this site. Much cheaper than Lee/Singh ray.

 

B&H as well as Adorama carry the Lee filter brand.

 

I've purchased Lee filters several times from 2filter.com.