What are the licensing restrictions for MySQL development?

Can you develop apps used in a commercial setting that use MySQL?

[dunkleosteus]

When do you have to pay MySQL AB for a commercial license? Say I want to create some php web based email mass mailing software that uses MySQL and wish to sell this... Is this permissible with the free license? What if I develop an internal app for a corporation? Would that be allowed with the free version?

[coopster]

The licensing FAQ [mysql.com] probably states it better than I can ;-)