This is a super easy-to-implement solution for SEO urls. Only two files need to be changed (as of version 1.0).
This requires an server with mod rewrite enabled.
Version 1.0 only rewrites category and product URLs, leaving reviews and other pages unchanged but hopefully this can be implemented in the future. This also only supports two levels of categories though this would be fairly easy to change if you need more. For most stores, this should be a quick way to vastly improve your URL structure for all products and categories.
Rewritten URLs will look like this:
While this retains the category, subcategory and product IDs in the URL (-c13 in the example above), this should have minimal impact on the SEO value of your URLs. Remember, SEO is primarily about the content on your pages, the URLs are just icing.