Google Custom Search Engine, available at www.google.com/coop/cse lets you actually create a search engine. You give it a name, a description, some keywords, a list of sites where to search, and you may allow other people to improve the list. You can restrict the search to the list of the sites you specified, or just emphasize them in the search results. The style of the search result pages can be customized and you can add a logo, like in AdSense for Search. The search results can be hosted on a Google page or can be displayed in a page from your site. You can even use Google Ajax Search instead of the standard formats. The ads displayed in the SERPs can be connected to you if you have an AdSense account.
The search engine can be improved using annotations (a set of labels that describe a page), that work like in Google Co-op, and can be added using Google Marker.
Here are some custom search engines from: Macworld, Intuit and a small search engine about Google created by me.
The new product is a combination of AdSense for Search and Google Co-op that makes it easy to create a search engine focused on specific topics.