Solr now does faceted browsing out of the box
For example, adding the following parameters to your query. assuming you have "category" and "inStock" fields in your data:
facet=true facet.field=category facet.field=inStock
Adds something like this to the search results:
Showing which facets exist for the category and inStock fields, including result counts for each.
More details are available on the solr-user mailing list.
This is way cool, and comes right on time for my current project!