Directive Studios Blogs
This variable ND filter from Heliopan is great for photography and shooting video. I primarily got it to shoot with a greater depth of field in bright sunlight when shooting video. The markings are very helpful and the quality is amazing. I've only played with a couple other filters, but this one wins hands down so far.
I also got a step up ring for my 77mm lenses and it works great. I haven't got to use it a lot yet because somehow I bent the step up ring. Just ordered another today.
Heliopan Website: http://www.heliopan.de/produkte/graufilter/
I read about it the first time from Philip Bloom (He's an awesome Director, DP and Fimmaker in the UK): http://philipbloom.net/2011/06/04/the-best-variable-nd-filter-i-have-used/