1. Blogs - to allow uers to contribute with comments, ratings etc.
2. Bookmarking/Tagging - to save links that users like
3. Brand monitoring - using blogsearch (http://blogsearch.google.com), social bookmarking (http://del.icio.us, http://technorati.com)
4. Content aggregation - Like http://popurls.com
5. Crowdsourcing/Voting - Like Digg http://digg.com, Reddit http://reddit.com
6. Discussion boards and forums - Like Slashdot http://slashdot.org
7. Events and meetups - Using Meetup http://meetup.com
8. Mashups - using tools and ideas from Programmable web http://programmableweb.com
9. Microblogging - Using Twitter http://twitter.com, Jaiku http://jaiku.com, Tumblr http://tumblr.com, Create your own Twitter using Laconica http://laconi.ca
10. Online video - Using http://youtube.com, http://blip.tv, http://vimeo.com, http://ustream.com
11. Online Organization and staffing - Using Basecamp, Campfirenow, Linkedin http://linkedin.com
12. Blogger Outreach programs - Approach influential bloggers to see whether they are willing to spread your love. Best to approach mid-tier bloggers.
13. Photosharing - Using Flickr http://flickr.com
14. Podcasting - Create for free using Audacity and host on your site and spread the link love
15. Presentation sharing - Using Slideshare http://slideshare.net
16. Public Relations and Press Releases - Using PR 2.0 sites such as Pitchengine http://pitchengine.com and blogging about incidents as they happen.
17. Ratings and reviews - Post the article on your blog, and submit the item to Digg, Reddit and get people to share their views
18. Social networks - applications, fan pages, groups, personas and CEo/decision makers' pages on sites such as http:;/facebook.com, Linkedin http://linkedin.com
19. Sponsorships - Sponsor contests on social networking sites
20. Virtual worlds - on Secondlife http://secondlife.com
21. Widgets - so that people can share/publish data from your site on their own
22. Wikis - Using Mediawiki, pbWiki or Wetpaint.

List inspired by Peter Kim's list