I have been a member of NPD Online Research for several months now. NPD is different from the other survey companies I have discussed so far in that for most surveys you are compensated in points which you can then enter in their "Sweepland" area. They let you know which sweepstakes you can enter into and you decide where and how to allocate your points into their giveaways.
From time to time, I get a survey with cash compensation and this is specifically outlined in your survey invitation. I receive approximately 1-2 survey invitations per week from NPD - they definitely do not flood your inbox! Like with all survey companies, I recommend having your own separate web-based email addresses just for surveys. That way, you'll have all survey invitations in one place and can log on at your leisure to complete the surveys you choose to do. Remember, it's FREE to join and you are under no obligation to complete any surveys.
Some of the other surveys companies already reviewed on this blog are Lightspeed, Greenfield, Global Test Market, Survey Savvy, SayNation, Valued Opinions, American Consumer Opinion, Opinion Outpost , Zoompanel, Ipsos I-Say, HCD Surveys, and university-sponsored sites. Have you signed up for all of them?