I don't make a fortune doing it obviously, but it is fun to get small checks in the mail for doing them. Here are a couple of the companies I have been completing surveys for a while now with.
I have been a member of Mindfield Online since January 2008. I receive several emails a week from the company. I always use my non-primary email address when registering and if I don't have time I can just delete the request. There is quick check to see if I qualify for the survey and they tell you how long it will take.
Another company I receive surveys from is Synovate Global Opinion Panels. I have been a member since July 2007. Synovate is great because it will reward you with points even if you don't fully qualify for the survey plus they have monthly sweepstakes for all survey participants. You redeem your points for cash. Again I registered using my non-primary email address.