It would seem that Sprint is on a roll lately with amazing offers and promotions. Their newest one is for existing customers and looks to be a brilliant way to retain people. Sprint is for a limited, and unspecified time, offering one free month of service if you renew your 2-year contract. Your contract doesn't even have to be up, nor do you have to get a new device!

It's important to note that this promo doesn't include anything added to your phone's service, like extra texting, etc., and it only works for primary lines. It basically refunds your primary charges. Here's a quote from the AndroidPolice article with a good example:
The credit applies only to the basic cost of your plan and doesn't include any fees or add-ons. For example, my $129.99 family plan ends up costing me about $170 after all add-ons. For me, only $129.99 would be refunded.
Kudos to Sprint! With all the great things going for the "Now Network" lately, it would seem that Sprint is rising to become the best of the bunch. They still have unlimited data plans, they offer great promotions for new and existing customers, their customer service was recently rated equal or better to Verizon's, their coverage areas are growing and expanding, and they have the cheapest prices. The list is nothing but positive! Keep up the great work Dan Hesse!

Source: SprintDroids via AndroidPolice