[mattstyle2]

Altered Cart's shop by filters is a decent base.. can be modified to filter based on whatever if you are good at php. seems to support 'extra fields' out of the gate too

for example, I made a gender filter.
added a column to the variants table
wrote a script that runs periodically to assign variants a gender, populating the variants table (based on if the item is in a sub cat under men's or women's or you can do it by a keyword in the title or keywords field for unisex items.
the filter will then look at the variants table and will only show products in stock if that variant has avail>0 and matches the gender being filtered on.. nice and quick.