1. Your current lender has some promotion, such as no cost no appraisal refi. But rate is usually higher than you can find in the market.
2. You work with a loan broker. Lenders usually give loan broker credit. Loan broker passes all or part of that to you to pay the closing cost. Loan broker can find the better rate for you (of course has to benefit them also by working with such lender).
At the closing table, you might still have to put some money in. Few things like following:
1. Interest on the balance of existing mortgage for the remaining days (depends on where the closing is done during the month)
2. New Escrow deposit your new lender requests.
3. County tax if it is due and wasn't paid by previous lender.
4. New home owner insurance policy if there is no enough dates remaining before the policy should be renewed.
After closing, your previous lender will return escrow money back to you if any balance remains. County tax money will be returned to you by title company if tax is already paid by previous lender.