http://www.houseparty.com/
One of my favorites. You sign up to host a "party" on their site (You'll see, they have tons. You review and test out new products in a party setting). They send you a party pack with free samples and promo items. I hosted a Jenny McCarthy -Your Shape for
Pssst
http://pssst.generalmills.com/
General Mills site. They send out free coupons (I got to try the yoplait smoothie in the freezer bag...yummy!) plus some good coupons to pass out to your friends. They also have lots of printable coupons to use or email to friends.
SheSpeaks
http://www.shespeaks.com/howitworks
Not a super active site, but my first free sample to try out was Ghirardelli peanut butter chocolate, so I liked it right away. Mostly a review site, but they will send out free samples for you to review and take a survey about. They also give you decent coupons with the product to hand out. Also has boards to post on, I haven't tried them out so much yet. TONS of reviews for all different products, a good place to check out before shopping! I recently filled out a survey and got a year subscription to Better Homes and Gardens.