As part of their ranking system, search engines judge your site by the number of other sites that link to you.
Getting quality backlinks without paying cash or backlash can be a chore. But remember, if you don't ask other webmasters to link to you they won't.
So be personal, offer a reciprocal link, and be sure to entice your link partners with pages on your site that contain information related to their sites.
Here's a tool you can use to check the number of backlinks to your site.
Link Popularity Checker
Also consider automating this task by creating a page of codes for banners and text links. Give your visitors a chance to self-serve links, and a percentage of them will.
Link to Me!