Social bookmarking, vertical search and desktop search are three areas that Google is working on to improve the user experience, it announced yesterday.

Desktop searching has been a hot area for the past few years, as people have found desktop search tools useful for indexing and retrieving information on their hard drives.

Meanwhile, social bookmarking services such as Yahoo's have become popular because they let users save links to websites they find useful, annotate the links and share them with others. Finally, vertical search is another emerging area, because it lets users look for web pages about specific topics and thus obtain a narrower set of results.

In social bookmarking and vertical search, Google launched a test version of Co-op, a service that lets users put labels on web pages about subjects on which they are experts. For example, a dentist could mark up web pages about dentistry, and users could subscribe to his labels.

Already, Google has signed up users to annotate web pages dealing with health and local events, but now that the service is open to anyone, users can label pages about any subject. The labels that users create will also help the Google search engine adapt its indexing and retrieval algorithms so it can deliver more relevant results. Those interested in participating in Co-op can sign up here.

Google also introduced a related social-bookmarking service called Notebook, a browser tool that lets users clip content from web pages such as text, images and links and save them to a 'notebook', which they can access from any computer and share with others. Unlike Co-op, Notebook is a single repository where a user can keep portions of websites he or she visits, along with notes. Google Notebook will be available next week here.

On the desktop search front, Google released a new version of Google Desktop, whose main added feature is its ability to run mini-applications called Gadgets that extend the software's capabilities. They will run within the Sidebar feature of Google Desktop. Google already has hundreds of Gadgets available, and it is also offering an API (application programming interface) so developers can create their own mini-applications. Google Desktop 4.0 Beta is available here.

Finally, Google launched a service called Trends, which lets users see how popular a search term has been over time, along with relevant news items about that topic. The service can be accessed here.

Jonathan Rosenberg, senior vice-president of product management at Google, called Google Trends an opportunity for users to "create your own zeitgeist". He and other Google executives, including CEO Eric Schmidt, spoke to journalists yesterday on Google's campus in California, where the company announced the services.