Browser Highlighter (from eBay) is an extension that will show you eBay listings with a click of your mouse for items that you are currently shopping for. By downloading the extension, the Highlighter automatically places a button on Web pages (at right) of certain shopping sites (Amazon, NextTag, Google, Yahoo). Click the button and you see eBay listings of that item, or similar items.
After briefly testing the tool, I find that it works best with very specific queries. For example, search "Chicago Cubs" on Amazon and you get books, t-shirts and the like. Click the eBay button and you get tickets for the upcoming playoff game. But if you search for a specific book title, you get more relevant results from the eBay button. Overall, it's not a bad way to see the current market value of a particular item or group of items.
Upcoming for the Browser Highlighter is a PayPal tool, where you can click a button to automatically fill in forms and pay for an item anywhere on the Web, a Skype tool for click-to-call functionality and a StumbleUpon tool that will show user reviews.