Amazon: With a .edu email address, you can get Amazon Prime Student free for six months, with a $10 referral bonus for each friend you can get to sign up. After your six months are up, Amazon Prime costs only $49 per year instead of $99.
AT&T: Check if you're eligible for discounts on a plan by entering your .edu email address on their discount page.
Verizon: Verizon makes their discount hard to find. Go to their discount page and scroll down to "Not yet a Verizon Wireless customer?" Enter your .edu email address where it asks for a "work address." They'll email you and tell you if you're eligible for a discount.