Total number of links from other pages to yours. Each time someone links to your site you get more credibility from the search engines. If the website linking to your site has a very high Google ranking, then you'll get even more credibility because they're already considered a trusted source.
Recommendations: Look for relevant websites to link to your site to give you more credibility such as:
“I wouldn't hesitate to use Spartan Spark again, and in fact continue to refer them to just about anyone looking for top notch development resources.” -Mitch Solway - President, Single Parent Love Life Inc.