Open Source Shop by Brand from MagentoGarden, http://www.magentogarden.com/shop-by-brand.html

Shop by Brand extension for Magento enables you to create separate pages for brands presented in your store and link to them from product pages, thus making it easier for your customers to browse and shop by specific brands. Instead of brands names you can use any category relevant to your store - such as manufacturers, artists, authors, etc. The extension can be applied to any number of single-select attributes simultaneously, so your customers can shop by multiple attributes. So, for example, you can have Shop By Brands, Shop By Color, Shop By Collection and etc. at the same time. This solution has been successfully used by a great number of major online stores for years and it's now available for Magento!

1. Automatically fills out the list of Brands, you don't have to do it manually;
2. Creates a neat list of all Brands in alphabetical order;
3. Brand could be set as "Featured Brand", which will be displayed as the a rolling slider at the top of "Shop by Brand" Page;
4. Build a separate page for every single-select attributes, including brand presented in your store;
5. Support Layered Navigation;
6. Allows shoppers to filter products by brand, with a categories sidebar shown on the page as well;
7. Allows for displaying brands only from the current category in the sidebar block;
8. Upload image for each brand at the backend;
9. Allow visitors to filter products by Manufacturers;
10. Easy SEO for Brand pages;
11. 100% SEO friendly URLs;
12. Separated HTML/CSS, W3C validated;
13. Easy to install, just copy and run;
14. Easy to use, easy to manage, easy to customize;
15. Comes with User Manual and Installation Guide;
16. Powerful tools to manage all the extensions.