We use Survey Monkey. Just a little tighter integration would be great. I can find customer responses in the system to follow up with them, but it's a pretty manual process.
SurveyMonkey is definitely the leader here. Would be great to have some basic integration so that when we run a survey we can send out an email, report on who clicked through (both of those obviously currently available) and then report back that specific users have completed the survey (this is what is needed). I'm sure others can come up with far greater needs, but this is the basic that would help me a lot.
I'm not too worried about having survey functionality in Marketo directly unless it provided the same level of functionality that you get in the likes of Zoomerang and Survey Monkey. Things that would interest me are a) getting an activity history that says someone completed a specific survey and b) being able to pass selected question values from the survey tool into Marketo. Obviously not all questions would be ones you would want to associate with a lead but some you would. For example questions that might relate to buying behaviour and enable targeted nurturing campaigns, or demographic information that you might not have on company size or industry (a type of progressive profiling). Don't get me wrong, having comprehensive surveying tools right in a consolidated offering within Marketo would be great (maybe an acquisition?) but the above are the types of things I would be trying to acheive.