Approval by a blogger may please a sponsor - NYTimes.com

Colleen Padilla, a 33-year-old mother of two who lives in suburban Philadelphia, has reviewed nearly 1,500 products, including baby clothes, microwave dinners and the Nintendo Wii, on her popular Web site Classymommy.com. Her site attracts 60,000 unique visitors every month, and Ms. Padilla attracts something else: free items from companies eager to promote their products to her readers.

Marketing companies are keen to get their products into the hands of so-called influencers who have loyal online followings because the opinions of such consumers help products stand out amid the clutter, particularly in social media...