Here are 10 free places to list your hostel online and get additional exposure on the Web:

1. World66.com
2. Wikitravel.org
3. Yelp.com (USA)
4. Google Local
5. Yahoo Local
6. Yahoo Travel — either find your hostel in the listings and look for an edit link or go to the home page and look for the suggest a hotel link at the bottom of the page.
7. HostelLinks.com
8. HostelGuide.de
9. Backpackers-planet.com
10. Add a hostel to HostelManagement.com

For tips on adding hostel listings to 3rd party Web sites, check out my tips on "piggy back SEO". Piggyback SEO will help your listings on 3rd party Web sites rank higher in search engines.

UPDATE: There is a now a list of 10 more Web directories.