One of the coolest sites I've found for this is Influenster.
Influenster has recently relaunched their website, but unfortunately, I was unable to see what the site was like before. I can only assume it was not nearly as easy to navigate as it is now. Everything is fairly easy to find, and it takes no time at all to get active and flowing so you can get your first VoxBox.
What is a VoxBox? It's a box of products they send you so you can try and review. It's completely free, but if you don't complete the required tasks (and some of the extra tasks), you won't get another box. Its a fair trade, if you ask me.
The boxes you get depend on the badges you unlock. You unlock badges by completing surveys and showing them you have access to more than one outlet on the internet (Facebook, Twitter, a blog (or blogs), a YouTube channel, etc.). The more vocal you are online, the higher your chances of getting a box become.
There are certain categories you'll probably never really get into, though, and that's okay. Not everyone is in college. Not everyone is planning a wedding. You might not be a gamer or a pet owner. The surveys and badges are how you tell them what products and situations are relevant to your life. Personally, I'd have no use for a dog-based VoxBox, as I only have cats. I won't ever earn a high enough rating to get a dog box, and I'm completely okay with that!
So many people out there are just interested in getting something for free. If this is you, Influenster is NOT the place you need to be. They've found ways to help limit how many "freeloaders" get boxes. If you're willing to do the "work" involved and can prove it to them on their site, you're gonna get a box eventually. It might not be immediately, but you'll be on their list! Think of them as a year-round Santa. :)