Great article on using extreme (or long exposure) ND filters here.
This list is not exhaustive, others companies do make long exposure filters.
Long exposure filters
Lee Big Stopper and Hitech 10 stop ND filter Review, here. Lee also now make the Little Stopper, a 6 stop ND filter.
Lee Big Stopper 10 stop ND verse B+W 3.0 10 stop ND filter review.
Lee Big Stopper 10 stop ND verse HiTech Firecrest 16 stop ND filter review. Lots of praise for the Hitech, not a great deal for the Lee BS.
Some initial thoughts on Lee Filters ‘Big Stopper’ 10 stop ND filter, here.
Formatt Hitech Firecrest 16 stop ND filter here and here. The Firecrest verse the Lee Big Stopper here. Both reviews speak highly of the Firecrest. There is also now a 13 stop version of the Formatt Hitech Firecrest.
Hoya also make a 9 stop filter, the Hoya ND400 (and also the ND16x). I have found no good reviews so far, but this is a helpful discussion on filters including the Hoya here.
B+W 100, 10 stop filter here.
Heliopan also has a 0.9 ND filter (8 stops).
PolarPro also has a rnage slot in and screw on ND filters.
Tiffen also have the Digital HT 16x (1.2) Neutral Density Glass Filter (Light Transmission of 6.25%)
Singh-Ray have an variable 2-8 stop filter, the Vari-ND Filter.
Wine Country Camera has 3, 6 and 10 stop ND filters marketed under the name Blackstone ND.
Variable Neutral Density Filters
Some circular filters are variable; the main ones are set out below with the range of each filter.
Aurora Aperture’s PowerXND 2000
Range up to 4 stops. You cannot use the lens hood as this filter has a built in handle. The product is directed more at video cameras, rather than DSLRs.
Lightcraft Fader ND mark II
Range 2-8 stops
Lightcraft Fader ND ULTRA mark II
Range 9-12 stops
Vari-ND filter
Range 2-8 stops
Vari-N-Duo (combined Vari-ND filter & LB Warming Polariser)
Range 2 2/3 – 8 stops
Vari-N-trio (combined Vari-ND filter, LB Polariser & colour enhancement)
Range 4-8 stops
Shoot out of variable filters here.
Phottix Variable ND filter (2-8 stops)
Lee Big Stopper verse Formatt Hitech and Haida here.