Sites that revolutionized the Web

I compiled a list of the websites that have changed the way information is produced, handled, or distributed over the Internet. Many of these sites have something in common: they enable users (human or software) to judge the relevance of their content, to provide "interesting" information to others. This has been called distributed cognition or collective intelligence: like diligent ants, the small contributions of many people or sites lead to great achievements and sources of information.

• Google. The most effective search engine for several years now. It judges the relevance of a webpage (its PageRank) by the PageRank of webpages that link to it. Thus, if many webpages link to a certain site, this should be interesting. And even more if those webpages are interesting themselves. Google has become an empire of free web-based utilities (in some cases buying other companies):

• GMail. One of the most flexible free web-based mail providers, it gives users almost 3Gb of disk space and decent spam filtering. A great tool for keeping and searching old mails.
  
• Docs & Spreadsheets. Edit and share online documents and spreadsheets. Great for collaborative work. Import and export to/from OpenOffice, Word/Excel, HTML, etc... (Used to be Writely, which Google bought...)
  
• Blogger. Well, the host of this blog and millions of others. There are a lot of blog providers, so I will not attempt a list. The benefit of blogs is that they enable the easy and quick production of information by anyone with Internet access. Certainly, lots of junk is produced every second (like this blog...), but out of that interesting things can be sifted... For example, the US Pentagon filters all information about Irak that is published or broadcasted on usual media. But they don't have control over the Web, so blogs became a powerful weapon against Bush's government attempt to hide information on US soldiers killed, prisoners tortured, etc.
  
• Books. Search on books.
  
• Scholar. Search on academic texts.
  
• and much more...
  

• Wikipedia. The world's largest encyclopedia. Wiki enables users to easily build web documents collaboratively. This led to the development by an enthusiast community of this free encyclopedia. Everybody can edit documents (which are of course monitored for accuracy), so its growth rate is immense compared to any other published by a much smaller group of experts. In less than five years, it has more than million and a half entries in English. Because it has articles in more than 250, from Esperanto to Nahuatl, from Swahili to Maori. The Wikimeadia Foundation has several sister projects, among others:
• Wiktionary. Dictionary and Thesaurus.
• Wikisource. Free content library. See also Project Gutenberg.
  
• Wikibooks. Free textbooks and manuals.
• Wikiversity. Free learning materials. See also Conexions.
  
• Amazon. Buy online! Largest bookshop in the world. Useful features include ratings and reviews of products by users, and automatic suggestion of similar products (people interested in what you bought, also bought...).
  
• eBay. Auction online! Users also rate sellers, so it is easier to trust somebody who has given excellent service to others. And it motivates sellers to give a good service, since they will be rated by their clients.
  
• Orkut, Friendster, hi5, etc... Social networking sites. Also useful to make new friends with desired characteristics. LinkedIn is similar, but business-oriented.
  
• del.icio.us. Make, share, tag, and search bookmarks online. "Social bookmarking".
  
• YouTube. Users can upload and rate videos. Recently bought by Google, which already had its own Google Video.
  
• Flickr.com. Store, share, search, and rate photos online.
  
• Sourceforge. Host of thousands of open source projects. Useful for collective development of software projects, since it provides free website, CVS, etc.
  
• arxiv. The first of several open archives, scientists can share their preprints before having to wait the long process of journal publication. Even online journals can take several months to review a paper. Best place to find out the most recent research, and the best way to divulge quickly your own.
• CiteULike. Make, search, share, and tag academic citations online.
• Digg. People vote on news items. News with most votes are shown in front pages, so you tend to get interesting news (for a certain category). Nowadays most news are tech-related, but the model has lots of potential. A similar site is Google news, only that the relevance of news is calculated automatically.
• Pandora. Free online radio, suggesting music similar to an artist or song of your choice. See this post.
  

If you have more sites to add to this list, please post a comment!