The Retirement Savings Credit is yet another "Wait! There's more!" feature of the tax code.
There are a variety of tax-favored retirement plans which can qualify taxpayers for this credit, which is a non-refundable credit.
Some of the most common types of retirement plans eligible for this credit are: Roth IRA, Traditional IRA, 401(k) and 403(b) plans.
There is more information on the Retirement Savings Credit in Pub 17 here.
There is also more information in the Instructions for Form 8880: Retirement Savings Credit.