If you have some independently-useful content on your website, such as a glossary of terms in your industry, you’ll probably already know that it can be a tremendous traffic generator. It might often be quoted as a reference by other sites too, which can be a source of links back to your site. However, not everyone will link to you as they should; you really need to encourage people to put the correct attribution in place.
A free service which does this is Tynt. Sign up for this, add a small piece of code to your site template, and when someone copies and pastes something from your site, the clever script adds a link to what they’ve copied. To see what I mean, go to the Tynt site, copy a paragraph of text, and paste it into Notepad or Word, and you’ll see what happens. Very cute.