InGroup is run by Henry Holt and is only open to teens ages 13 to 19 years old who live in the United States. There is a release form your parent or guardian needs to sign that needs to be snail mailed before you can join. But once you're in, you get an email once a month with three or four book titles. You pick one you are most interested in reading and if you're one of the first 25 to get your email in for that title, they will mail you a copy (I received two book from them already). You read it and fill out an easy feedback form with seven questions. They might even quote you on InGroup website.
I joined the Harlequin Teen Panel in May and I'm still learning how this program works. The only emails I received from them are surveys about book titles and covers, but they are really fun and quick. There were also discussions and author forums I took part in. According to their website, you're to receive free Harlequin Teen books by joining but I haven't gotten any since I've joined so I'm not sure about that. The program is fairly new though so I guess they are still working out a few kinks. Like InGroup, a parent has to fill out a release form but it's done entirely online through emails and it's only open to teens ages 13 to 17.
My sister and I have been part of Random Buzzers the longest. It's a great reader community where you can take part in fun quizzes, author forums, fantastic weekly giveaways, post reviews, interact with other readers and authors and other fun things. Just about everything you do on the site earns you "buzz bucks" and when you accumulate a certain amount, you can exchange it for books. There's no age restrictions but I'm not sure if it's open worldwide.
My sister signed up for NetGalley a while ago and she doesn't use it because we don't have an e-reader and it sucks reading on the computer for many hours. However, we did browse their catalog and there were a whole lot of fantastic e-ARC titles on there in all genres with many different publishers offering their books. You can request a title and download it directly to your computer or e-reader. There is a time limit to the e-ARCs though--about 2 months I believe but it's open internationally.
These programs are all awesome in different ways and if you are interested in alternative ways of getting free books to review or feature on your blog, they all are worth checking out. I know there are other book programs for teens (and adults) out there but these four are the ones I am most familiar with and I wanted to let you all know about them. If you are part of other book programs or know others, please share in the comments below.