You can get started with either UserVoice or GetSatisfaction for free.
We're using UserVoice, because we found it works really well for a forum where your users can post feature requests and bugs/problems with your app. Right now, we are on the UserVoice free plan, but we will probably upgrate soon to a paid plan.
The one really big feature that's missing from the free plan is the single sign-on feature. This enables a user that is logged into your webapp to remain logged in after they click on the Feedback tab and go to the UserVoice support forum. The problem I am seeing is that users click on the Feedback tab, go to UserVoice, and post feature requests as Anonymous. This is problematic because there is no way for you to tell which user posted the feature request, and there is no way for that user to see your reply (unless they go back to the UV forum to check, which they probably will not do). If you know who posted the feature request, you'll also have the option to contact that person via email and have a back-and-forth conversation about his/her feature request.
If you're running a consumer site, you might not care to reply directly to specific users. We're running a b2b site, and we definitely want to reply directly to specific users posting feature requests.
UserVoice offers the single sign-on feature in their $89/month plan. GetSatisfaction offers that feature only on their $279/month plan. Obviously, the choice is clear.