Yahoo (400 mio user) and Ebay (200 mio user) are partnering in online advertising, online pay services and online communication to improve their position against Google and Microsoft. Yahoo will be the preferred search engine in Ebay and will be integrated in the Ebay toolbar. Ebay on the other hand will be the preferred service provider for Yahoo-online pay services.
Skype will also be involved in the cooperation, but no details have been announced yet. But in my opinion, it would be a great feature, if VoIP is integrated in search result (e.g. if with search results a "call-button" would be displayed)