Most people involved in SEO are familiar with primary link sources such as directories, press release websites, blogs, forums, article syndication websites, social networks and bookmarking portals. However, as part of your link building, consider the following alternative sources that can provide you with much needed link juice:

Customized 404 Pages
Just be sure that missing pages on your website are not impacting inbound link numbers. If someone links to your website and they get a 404 error, you may be missing out on potential incoming links. Be sure to address this problem. If you have your own custom 404 error pages, do a crawl test (available in Google Webmaster Tools) to identify broken links and put in place permanent 301 redirects to appropriate pages on your website. Oh, and do test the links individually after you implement the 301 redirects.

Testimonials
It may be worth contacting some website whose products & services you use and discuss the possibility of you providing them with a testimonial in order to get an inbound link from them. This may take a bit of work but the results are well worth it, especially if you can get a link from a highly ranked website.

Wiki’s
Apart from Wikipedia, there are a large number of other Wiki’s that cover a very large number of topics. Links from Wikis are beneficial and generally tend to be in-content external links that appear within the body of an article, or content. Search Engines assign a high value to such links despite the fact that some of them place “nofollow” attributes. Some examples: Google’ sKnol and AboutUs.org.

Free Press Release Websites
SEO experts are familiar with PRWeb.com. However, there are several other free press release distribution websites that can provide you with high value links. Some of them don’t allow the creation of anchor text links but these websites are indexed and do appear in search engine result pages. Some examples are 24-7PressRelease.com, PRlog.org and PR.com.

Social Network Profiles and Pages
A lot of social media websites allow you to create custom profiles using your name, or that of your business. Take advantage of this. Do remember that idle social media profiles can do more damage than good so be sure to do periodic updates. The frequency of the updates will depend on you business and on your service and/or product and industry.

.edu pages
Links from educational organizations are highly trusted by search engines. Although difficult to obtain, you can create a profile on your educational organization’s alumni section, or you can create a profile on a sorority, fraternity or educational related volunteer website that may be linked to your alma mater.