For a limited time, you can grab an extra three months of Xbox Live for free when you purchase a three month subscription at Amazon for its regular price of $24.99. This is a match of a deal which GameStop and Best Buy offered earlier this week (though GameStop did additionally include a download of Rocket League for a short time).
$25 for three months of Xbox Live is what you'd generally have to pay for that subscription length, though six months has always been discounted a bit at $40. This deal saves you $15 off that card's regular price but would let you share the extra 3 months with another account if you'd like.
Xbox Live members receive free games every month, along with Microsoft store discounts and more.