We've defined subject directories as sites that select a limited number of websites or pages and allow you to browse them by topic in a logical, hierarchical way. But there are a number of much less formally organized, but still very useful sites. Instead of coming up with a hierarchical structure, they simply "tag" materials, the same way users of YouTube or Flickr do. There is no one word to cover this kind of site, but you can think of them as "filters": they are taking the incredible volume of material available on the Web and filtering out the noise.
Docuticker, for instance, notes interesting freely available reports from a variety of sources as they appear. If you are doing a topic on relatively recent events, this can be a great site to explore. You can use the search box at the top of the page, or click on tags listed on right-hand side.
Free Government Information tags documents from government, educational and non-profit sources in a similar way. A certain amount of the content is commentary unsuitable for citing in your work, but the links can be extremely useful. Check out their page on censorship.
If you want to hear a lecture, try Princeton's UChannel (also available as a YouTube channel). This site collects video and audio presentations made on college campuses across the country and tags them according to the topic. You can stream them and, in some cases, download them to your computer or a portable device. These talks can be a great resource; you should, however, take note of the speaker and consider (as always!) questions of authority and bias. Is the speaker a scholar? a journalist? a government official or diplomat? Also, before citing this material, check with your professor to see how s/he feels about citing audio-visual content.
You might stumble upon a site that looks similar to these while searching the Web, or hear about one from a professor or classmate. They can work out very well, but always investigate who is responsible for the site and look at a few examples of what kinds of material it links to.