Idea Informer provides a public way to collect visitor feedback
Idea Informer is a widget that you put on your site, and it takes the form of one of those "feedback tabs" to the right of the page (you've seen them before, I'm sure).
The thing that's a bit different about Idea Informer is that any comments left by visitors become public, and other visitors can vote them up. You can think of it as a public wish list. One problem with the implementation is that it doesn't require you to log in to vote; it also lets you vote on anything and everything.
Dropbox implemented a similar system for internal use so that users can vote for features that they want to see. But what's neat about Dropbox's system is that you only get six votes at a time, so you need to choose wisely. With Idea Informer, I can click on anything I like (once per item). I actually went through an entire list of someone's feedback and added one vote to every item, just to make sure it really works (I didn't skew the ratings, though – I've kept it all even, as it was before).
You also do not have to be registered to leave an idea for people to vote on, but it does ask for your name and an email address, and it presents you with a CAPTCHA prompt.
All in all, I've seen better implementations. Get Satisfaction is one that comes to mind. However, I still have a feeling that, if its makers tweak it a bit, Idea Informer can be a valuable tool.