A lot of nonprofits use SurveyMonkey because it's the "market leader" in surveying. It's OK, though its nonstandard, inaccessible form elements can be an issue. More significantly, it caps the number of questions you can have in each survey (10) and the number of responses you can process (100/month) unless you get the paid version — and at $20/month, it's not cheap.
Enter SurveyBob, which allows you to create unlimited surveys, with unlimited questions, processing unlimited numbers of responses, for free. The statistics aren't as deep as SurveyMonkey, and at the end of a survey it dumps respondents onto the SurveyBob page, but still looks more professional than SurveyMonkey. Allows for embedding directly in websites, too.