Bookmarklets are bits of code you can drag to your browser toolbar to allow you to advanced search on search engines. Here are some I find useful:
Google:
Cache of Current Page (All browsers)
This Favelet/Bookmarklet lets you instantly see the Google cache of
whatever page URL you are on at the time of selecting the bookmarklet.
It is useful for checking whether Google has indexed a page, or for
spotting whether recent changes have been picked up yet.
Google:
All indexed from site (All browsers)
This Bookmarklet/Favelet opens a new window of Google search results
that enables you to find all of the pages from whatever site you are
on that are indexed within Google.
Google:
All mentions of site (All browsers)
This Bookmarklet/Favelet enables you to find all mentions of whatever
domain you are on that are on pages indexed within Google.
Google:
Backlinks Sample (All browsers)
This Favelet opens up a new window showing some of the backlinks that
Google has indexed for the site. Google rarely shows any links from
pages that do not have a high PageRank score in this filtered sample
of backlinks.
Yahoo!:
Find Backlinks (All browsers)
Find all links to any part of the domain you are on that are known
to the Yahoo! search engine.