Most online retailers already signed you up to receive their free email newsletters by default when you checkout. There's usually a little box on the checkout page that asks if you'd like to receive news of special offers and mailings. Unless you notice and uncheck the box, you're on the mailing list.
I'm on a few of them: Amazon, Eddie Bauer, REI. While it's interesting to get notice of a new hardcover release, these emails are temptation marketing. Sure, I click on the link and go browse. But unless the offer is something astounding (free shipping, take an additional 30% off, etc.) and I'm already in the market for these items, it's a distracting temptation.
This morning I had two retailer emails and I went out browsing both. I even had added two items to my shopping cart. Then I realized I was buying on impulse and no markdown would compensate for spending money on something I didn't even need and bailed out. Close window. Delete the email.