There have been many studies, and much anecdotal evidence, and every has their own opinion of how to motivate employees. I'd like to hear yours.
My own sense is that people are motivated by the following in a workplace, and not necessarily in this order.
1. Camaraderie. It's hard to love an abstraction like the legal fiction of a corporation. At the same time, people like to feel part of a team and part of something bigger than themselves. It's not difficult to come to work if you really want to see your co-workers. And people who feel esprit de corps are willing to achieve business goals for each other, if not for the entire organization.
2. Ambition. People have specific, personal life goals that they'd like to achieve. This can be anything from a deeply held sense of vocation, e.g., the service to others you see in a nurse or in a priest, to a financial analysts' dream of running a successful and acclaimed Wall Street fund — or firm.
3. Appreciation. People love to be told they are doing a good job, especially when are doing one. Even the most cynical person will find some satisfaction in recognition.
4. Compensation. Professionals need to be paid salaries and have benefits commensurate with their contribution to the company. One corporate owner I know (not my current company) simply cannot understand that appreciation cannot take the place of a paycheck. He recognizes people well, but then is a skinflint come salary review. Consequently, he gets a lot of people who feel "played." Even God Himself said it, "Where your treasure is, there will your heart be also." That is, what's really important to you isn't what you say, it's what you pay for.
5. Growth. People love to learn. They hate to be taught, but they love to learn. Even the best compensated, best appreciated executives will get bored if they don't have a challenge before them. Contrarily, companies that offer professionals opportunities to build skills, try new things, and a workplace filled with smart people sharing ideas and information, won't have a hard time retaining the right people — and will be better prepared to deliver business results.