- work on something they find worthwhile
- work with great people
- work in a management system that lets them do those things without lots of hassle (bad management systems - see Dilbert for lots of examples)
Post on building a great team
As far as motivation, if you hire and manage right this isn't something you need to do. You need to eliminate de-motivation not to worry about motivating employees.
In response to Clarity question: What's an alternative to equity based compensation that recruits, motivates and retains employees?
Related: Let employees take pride in their work by eliminating de-motivation in the management system - Hiring the Right People