Hoya. Hoya FUSION antistatic filters here.
K&F Concept filters, a very cost effective way to enter the world of filters.
The PRO Polariser, by Dean Cooper. It is a circular filter fitted to a frame to fit into a square filter holder.
PolarPro Summit Filter Kit, review here. Premium priced product, but includes filter holders and key filters. Filters are proprietary to the PolarPro system, so you must buy all your filters again. Wine Country Camera at least allows you to use you NiSi and Lee filters in their system.
Manfrotto Filters
NiSi Square Filter System here and here. Review here, another review here (not so positive). A recent review by Dean Cooper is far more positive, he reviewed the new version 5 of the filter holder. Dean Cooper knows a lot about filters (The Doc went on one of his NZ Workshops, which was fantastic).
Rodenstock Filters here, here and here
Sigma has brought out a new ceramic glass filter here.
Wine Country Camera (the Buggatti of filter holders, including the price tag), review here. The Doc has bought into the WCC system when it was on sale, using my Singh Ray and Lee filters in their filter holders.