Jellybooks offers readers the ability to sample 10% of an ebook and then forward the sample to her/his friends via email, Twitter, and other networking services. Registration is not required, but is encouraged through discounts to purchase ebooks at third-party retailers (including Amazon KDP Shop and Apple Tunes store). Membership is free. Jellybooks claims it won't allow onsite advertising.
Benefits to Registering (from Jellybooks' Help pages):
Jellybooks states it's only interested in publishing partners that list 50 or more titles (http://www.linkedin.com/company/jellybooks).
Check out Don Bodie's overview of both Jellybooks and Bilbary: