Dabs like product filtering

Hi,

Im trying to create a search\filtering similar to dabs, see link below.
So the “Refine Your Search” menu breaks down the different
categories\sub categories for a product with a count of how many are
available and you can drill down…

Is there a plugin in rails that makes this easy(ive looked about but
nothing seemed to meet this requirment)

or

is there any advice on how best to do this.

Options maybe to do it dynamically so when a customer searches then get
the relevant sub categories and do the counts seperately(this seems
intensive and will kill performance as the dataset gets bigger)

Another option may be to do it evytime a new product is added to update
a table with counts etc, again this seems a bit overkill as it will need
to be done everytime a product is added/edited etc

Any advice appreciated,

JB