Everyone knows that Yahoo and eBay have geared up for a merger. The combined company (YahBay, eHoo, or Boo.com) could really battle Google on most fronts. Of course, neither Yahoo nor eBay can beat Google's search engine, but Yahoo's okay with being #2. Yahoo could wrap ads around that money-sucker Skype, and eBay could integrate with all of Yahoo's other shopper-friendly content.
On the other hand, why can't Google buy eBay? That'd give them an instant user base for VoIP, online payments (Contextual ads + PayPal = major commissions), and net commerce. Both of those gains would shut up the critics who predict that Google's ad-based revenue stream will dry up with clickfraud.