FORTUNE — Fierce competitors need, well, fierce competitors. The latest smack-down? Oracle (ORCL) billionaire CEO Larry Ellison vs. Salesforce.com (CRM) billionaire CEO Marc Benioff. Benioff once worked for Ellison, who later invested in Salesforce.com, the cloud-computing software company. No matter: Now the two battle for customers — as well as bragging rights as the world’s most visible software mogul. The rivalry is getting testy: Recently Ellison canceled Benioff’s keynote at the Oracle Open World conference.
This article is from the December 12, 2011 issue of Fortune.