One of the best link building tools that I have found has been through my e-commerce sites. It goes to building the relationships with the people you do business with. Here is my approach. Most online E-commerce businesses have suppliers, suppliers have manufacturers and other business associates. I will use one of my sites as an example: Marine parts, I have numerous wholesale suppliers. Most have links to the product manufacturers. I have to go to manufacturers for permission to use images, I also ask for links to specifications. The manufacturers I have are reputable and have had an internet presence for quite a while. Examples would be Teleflex, Morse controls, OMC, Mercruiser...all well known and reputable. My suppliers also have links to images. rather than swelling my website with image data, if, a supplier will allow me I use the link data. I happened to stumble onto this because on one supplier I had overlooked removing links and my ranking had shot up, I performed an analysis of my site and discovered all of the links were present, so I checked with the supplier and they said of course that's what the link's are there for, if you succeed we succeed.