The scuttlebutt on the GigaOm and TechCrunch blogs this week is that eBay (NASDAQ:EBAY) is gearing up to pay $40 million to $45 million for social bookmarking specialist StumbleUpon. Rumored bidders in the past included Time Warner's (NYSE:TWX) AOL and Google (NASDAQ:GOOG).

Never heard of StumbleUpon? Read on. The company provides a toolbar application that has been downloaded about 2.2 million times. Once installed, users rate sites as they surf the Web, clicking on the TiVo-esque thumbs-up and thumbs-down toolbar buttons. At any point, users can click on the StumbleUpon icon and be sent off to a page approved by fellow users with similar tastes.

It's a small company with a small price tag -- and big implications. It's easy to see what eBay can do with the popular toolbar. A community that rates auctions, hooks up on Skype, and settles transactions through PayPal can easily enhance eBay.

The surprise here is that the search-engine giants aren't beating eBay to the punch. Toolbars that navigate online experiences could threaten companies like Google or Microsoft (NASDAQ:MSFT). Controlling the desktop experience also makes Microsoft's Internet Explorer even less relevant.

Yes, StumbleUpon is small. So what? The toolbar application, in the hands of a dot-com juggernaut, can grow into a monster fairly quickly. Buying StumbleUpon is almost as important at this point as keeping that same ammo out of the competition's grubby hands.

For more on eBay, which just reported earnings:

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Longtime Fool contributor Rick Munarriz is a satisfied eBay user, with 171 positive feedbacks to show for it. He does own shares in TiVo. He is also a member of the Rule Breakers analytical team, seeking out the next great growth stock early in its defiance. The Fool has a disclosure policy.