Limesurvey is pretty good in terms of cost-effectiveness. If payment is an option, then qualtrics is good for nice layouts, and the ability to correctly re-format surveys on tablets/phones/etc..
We have used Survey Monkey. We find it easy to use, and we feel we get as good a return from this site as any other. I did, however, enjoy the discussion on the other sites, especially the ability to format the survey for tablets or phones.