A vintage, sine-wave, analog tremolo, made with carbon comp resistors. It can also be used as a clean boost! Available at JetCityMusic.com.
http://www.jetcitymusic.com/index.php?page=shop.product_details&flypage=vmj_naru.tpl&product_id=353&category_id=46&manufacturer_id=32&option=com_virtuemart&Itemid=89