I added a Google custom search engine to my web site this weekend. You can always get Google to limit it search to a particular domain by starting your query with
site:domain, but custom search engines are more convenient and more configurable. You can specify which sites to search, so you could combine multiple domains or specify individual pages. I configured my search engine to search my domain as well as several articles I’ve written that are posted on other web sites.
I also created a custom search engine for my employer’s software download site. The site’s built-in search feature only indexes project descriptions. The custom search engine indexes everything Google can find on the site and so it includes reference material such as product manuals. The two are complementary: the built-in search returns a list of products while Google returns a list of pages.