UserReport does! You don't have to do anything.
To encourage people to participate in the UserReport survey on your website, all respondents are invited to participate in a monthly prize draw conducted by UserReport. By the end of the survey, the respondent is asked if he/she wants to participate in the prize draw as a token of appreciation for the time spent taking the survey.
It's optional for the respondent to participate and UserReport contacts the winners directly, carries the cost of the prizes and takes care of all handling. The prizes include gift cards and the like.