I've seen a lot of promos from mobile carriers, but Sprint's promo is the best ever.

For a limited time, existing Sprint customers can take advantage of Sprint's Unlimited Line on Us promo. Simply add a new line of unlimited service for $0/month and Sprint will pick up the tab for the new line as long as you are a subscriber. (You will need to log into your account to begin the process). The free line includes unlimited calls, texts, data, and 480p video streaming. By comparison, Sprint's unlimited Kickstart plan starts at $35/month, so you save at least $420 in just one year.

The only caveat is that you must maintain your Sprint account in good standing. Also, this deal is only for existing Sprint customers. Otherwise, you can reserve a free line now and add a phone later.

As a reminder, the T-Mobile/Sprint merger was completed earlier this year. The merger does not affect Sprint's contract at this time. You can switch your line to T-Mobile when the time comes. In addition, the merger will put Sprint and T-Mobile in a better position to compete with Verizon and AT&T.

This deal with Sprint has no expiration date, but it is unlikely to last long, so take advantage of it while you can.