Short FAQ on the topic:

Q: Which sites can be added?
A: You may add any site currently powered by open source content management system Drupal.

However, it is not in our policy to list sites with excessive adult, inflammatory or abusive content.

Q: How do I add my site?
A: Register an account with Drupal Sites Network (it's absolutely free). Just be sure you are logged in. (You may use your Drupal.org login information or login info from your own site if you have drupal.module enabled.)

Click here to submit your Drupal site!

(Or just click submit drupal site! from your navigation menu. Remember, you have to be logged in)

Please do not submit sites not powered by Drupal. All new additions will be moderated.