I received a great sample in the mail the other day of hot chocolate. The best way I have found to get free samples and coupons is to "like" couponers and specific brands on Facebook. Everytime you look at your home screen, it will show the most recent deals they have added. You can probably also do it on Twitter, but I never got into that.
I never keep track of the samples I send away for, because it usually takes a few weeks. By the time I get it, I've forgotten about it, and then it's like getting a fun present!
Here is a list of some of the sites I follow:
If you have any that I didn't include, please post them in the comments.