I have an E-commerce site where I sell automotive accessories. Checking recently on Google Webmaster Tools, I am informed that links to my site are showing up on hundreds of sites that have nothing to do with my product (religious sites, artists sites, law firm sites). Also, may of these links have the anchor text of things like "find discount viagra online" and "no prescription viagra". When I tried to navigate a few of these sites to find my links, I was unable to find them. Where are these links coming from? My first thought is that a competitor is out there trying to get me a penalty from Google for link spamming. Is this a common practice?