BookMarklets, Favelets, and Javascript Bookmarks
Bookmarklets are bookmarks with some javascript code that can perform a task for you when clicked. Common uses include sending the URL of the current page to a service (like Murl or Bit.ly) or extracting information from a page.
To use these, drag and drop them to your bookmark toolbar.
Amazon URL: Redirects the browser to the current Amazon page but with a referral tag added. Drag and drop this to your toolbar and edit it to put in your tag (or not).
URL Lister: Pops up a question, then returns all URLs on the current page with given extension. Very handy for getting a list of all mp3 URLs.