I've been having a lot of success with Opinion Outpost lately! I've been receiving 2-3 survey invitations daily, and qualifying for at least 1 of them! The great part of Opinion Outpost is that you only need to earn $5 to be able to request a check...unlike so many of the other companies that make you earn $20 or even $30. If you'd like to sign up and start earning (they do NOT spam you--the only email I ever receive from them are notifications when a new survey is available) click here.
There will be some profile questions you have to fill out (so they can send you surveys that you are more likely to qualify for). You will probably be able to EASILY make an extra $30-$50/month with them. Good luck!