Firefox
Greasemonkey hack:
sri-goog-amazon.user.js. (make sure you know what you're doing if you install this...)
Select a piece of text with your mouse and hit one of these keys:
- ALT-A: searches Amazon.com for the selected text, in a background tab
- ALT-G: does the above with Google.com
Also, without any selection, ALT-A and ALT-G will take you to Amazon.com & Google.com respectively.