Bookmarklets is a very nice way of doing small (or not so small) things in/with a web browser, and is just a
(oneliner) Javascript script saved as a bookmark.
E.g. here is
a simple bookmarklet for viewing the HTML source of a web page.
If you bookmark the link, you can use it later on whatever site you want.
The Javascript code for this bookmarklet is:
bokus <-> amazon.
This bookmarklet searches for an ISBN on a page (on the form ISBN: 0201310058) and shows the amazon page for that book. If the page is on amazon it
search for the book on the best swedish online book saler: bokus.com. This last feature may only be useful for swedes (as bokus.com sells only in Sweden). I use this bookmarklet very often myself.