At Rudy Karsan's company, people grin at one another all day long. He's the founder and CEO of Kenexa, the leading human-resources-services company in America. Sixty percent of the Fortune 100, including Caterpillar, General Motors, Time Warner's, and Wachovia, hire Kenexa to help get inside the minds of their employees and build worker loyalty. Karsan's firm has pioneered new ways to blend psychology and technology. But his real secret is much more simple: A conviction that employees just want to have fun.