One interesting way to build a customized search experience is by using linked Custom Search Engines (CSEs). With linked CSEs, you can create a dynamically defined search engine that can be updated automatically. As an example, you can check 2008 Summer Games site, Custom Search box in the upper right corner that will offer you results from a set of sites that cover the games.
A cool application to create a personalized search over your del.icio.us bookmarks is described in the tutorial Build a Custom Search Engine using your Social Bookmarks. You can export your bookmarks to define a CSE and search across all the stuff you care about. Many social bookmarking sites make this kind of application easy to build by providing tools for exporting bookmarks or by listing all bookmarks on one webpage. Services like CSE builder application provide similar functionality.
You can create a CSE on the fly using the MakeCSE tool. Provide the URL of a page that has a bunch of hyperlinks and MakeCSE will extract the links, construct a CSE, and provide you the search results instantly. As the links on the page change, the CSE is automatically updated.