Advanced Navigation by GoMage. Works perfectly on Magento stores and the latest versions are optimized to work with the newest Magento releases CE 1.9 and EE 1.14. Integrated Ajax filters make the search on the site real quick and convenient. It is also SEO friendly and fully customizable as it has open source code. Besides, GoMage support always is there to help if you have any problems. The extension usually costs $119 but they offer special discounts in December, so it is now only $113.05. They also have a lot of different extensions for other purposes, as well as additional Magento related services. Just check their site at https://www.gomage.com/