SEO (Search Engine Optimization) is based largely on crosslinking. That is, every link on the web pointing to your site increases your chance for a higher page rank.

For an example Loopia (my domain provider) just setup a competition in which signing up is made up of everybody posting a blog entry with a link to the competion:

This is a neat trick making hundreds of people link to Loopia in a SEO-“valid” context improving their pagerank. And as a bonus, they post all pingbacks on their site effectively giving me a link back. Greyt!