Another method that reduces the overhead required by your users even further is to use online polls. Online polls suffer from one major problem though, unless you already have a username/password system set up, you will have no way of guaranteeing only one vote per person. You can take steps to reduce vote abuse, but you can never eliminate it altogether. Despite these weaknesses, online polls have significant benefits. The information you can acquire by getting poll results to properly formed questions is priceless. Online polls also make it much easier for visitors to provide information, normally requiring only two clicks. Finally, people also tend to enjoy taking online polls since it allows them to compare their opinions to those of other visitors.
The questions you ask will be of prime importance when building your polls. You must make sure that your question is short, clear and precise. The answers to your poll must also be short, clear and precise. But they must also be very clearly distinct from each other. With proper polls, you can gather reasonably reliable data on your website's audience.
An online poll will take time though since you'll have to leave a poll up on your website for at least a week to get a reasonable cross-section of your visitors to participate. Also, keep in mind that if you begin to post a poll every week, people will expect to see a new one every coming week. This won't be an issue if you have a lot of time to put into your website, but if your website is a side project or if you only have three or four questions that you would like answered, then this visitor expectation could cause you problems. My suggestion would be to spend some time building a small catalogue of useful polls and automate their deployment to your website on a weekly or monthly basis.