Dual licensing is fairly common. Then there's also nag messages that asks the user to pay. Then there's also "free for personal use". The thing is though, that

Dual licensing is fairly common. Then there's also nag messages that asks the user to pay. Then there's also "free for personal use". The thing is though, that people don't pay unless they have to. In my experience nag screens are not that effective, but adding profanity might make it more effective :P If you make it "I CAN'T AFFORD TO PAY $10 FOR THIS SOFTWARE" or something, businesses will want to get rid of it.