We built something in-house (Rails), and have been thinking about spinning it out into a minimalist feedback app. I'd be happy to share the code with you if you give me feedback on it. charlie@monotask.com
Regarding currently-available services, I've also used KISSInsights and Olark. Both are useful, and are ready to go right now. I'd check them out, but recognize that they have a fairly specific focus (not a bad thing; just might not work for you). KISSInsights is like a really minimal survey. It's really great for that. I believe they have an option where users can give you open-ended feedback, but I don't know if that's in the free account. Worth checking out. Olark is good, too, but I've found it's only useful for synchronous interactions. If I don't actively engage with people on it, it doesn't yield any substantive feedback.
All that being said, check them out.
