Competitive intelligence is an effective and therefore widely used link building technique. Look at whose linking to your competition and you'll find new link sources, partners, ad placements and get insight on the link building strategies of your competish. Howver search engines will only show you a fraction of incoming links to a site - no more than 1,000 links per domain. Now there's a way to bypass this limit and dive deeper into the pool of the hidden link sources. There's a new version of SEO SpyGlass (backlink analyzer) that bypasses the search engines' limit of 1,000 links per website and gets 6k to 8k incoming links to any site! Just tried it myself and it does work I ran a test for google.com and SpyGlass dug 6,140 links (no duplicates). So if you want to get some link sources to tap into go download the free version (http://www.link-assistant.com/seo-spyglass/download.html). For those of you who've never heard of SEO SpyGlass before, it's a backlink analyzer that allows you to check incoming links to a site and analyze them according to a number of important SEO factors: Google PR, dofollow/nofollow, link value, anchor text and a whole bunch more. This way you can look at where your competition get their links, get some link building ideas, find new link sources or analyze your own links.
SEO Spyglass is one of my FAVORITE SEO tools on the market today. I can attest to the above information - great product and great service.