7. Google and Yahoo

Frenemies since: 2000, when Google, which was not yet a goldmine, signed a profitable deal to provide its search engine to Yahoo, at the time was more Web directory than engine.

Acts of friendship: The search deal continued until  2004, when Yahoo ended it to launch its own homegrown engine. And in 2008, Google rode to Yahoo’s rescue in the wake of Microsoft’s hostile takeover attempt by inking a major deal to sell ad space on Yahoo (it later backed out due to antitrust concerns).

Acts of enmity: Even while Google powered Yahoo search, Yahoo’s Overture subsidiary was suing Google for ripping off its ad-serving technology (the companies settled in 2004 in a truce that gave Yahoo an ownership stake in Google). Then there’s the little fact that Google’s massive success has left Yahoo struggling to figure out how to be a strong number two in search.

Current state of the frenemyship: It’s Google’s world–Yahoo just lives in it.