The best feedback you’ll ever get about your business. Free.

It takes some courage to open yourself up to anonymous feedback from customers -- some of which may be unfair criticism from someone just having a bad day -- but the rewards far outweigh the risks. It shows that you're committed to providing a great experience and that you value and respect your customers' opinions. You could even go a step further and post some comment cards -- with your response -- on a message board of sorts. I saw this at my company's cafe once and it helped me feel like they were really listening and acting on suggestions.

I agree wholeheartedly that customer feedback, whether solicited via a formal card or web form, talking to your customers (at a bar/restaurant), or simply observing their behaviors (a la web analytics or with eyeballs) is one of the most valuable tools a business owner can use to improve their product or service. Successful business owners know this (or will learn it from reading posts like yours), and that is part of what makes them successful.

A lot of this is really just opinion, but the basic idea that you should listen to your customers is still good advice.