Web-sites such as news.google.com can provide a more personalized presentation to visitors by applying a simple sorting technique.
Long lists of information--principally of Web-links--can easily be sorted by probable interest so that links a user may be most interested in gravitate toward the top of the page.
Google's news page is a good example to explain how to apply this technique. (http://news.google.com/)
Google's news page lists news article links from many disparate yet related sources--from traditional new sources such as Time and CNN to online news such as Salon.com and Slashdot.org. The list is sorted in (what appears to be) an arbitrary manner.
There are two levels to Google's sorting approach; links are categorized first--Political, Entertainment, Science, Sports, etc. Each category is sorted by publication time, latest first. What Google has (intentionally or not) basically applied a weighting factor to each link: Category and Time. By further weighting with keywords--something Google has capability of already--Google can simply maintain a selection history for each unique user, and use it to sort by this weighting factor.
For example, if we were to look at my viewing history, one would find that I rarely view articles categorized as Sports and Entertainment, and mostly view articles categorized as Science and Politics. If there were a list of keywords attached to each article-link Google would have a measure of what kind of articles I mostly view.
Google can then sort the articles list to my probable liking--articles most likely of my interest at the top.