Just wanted to point out to anyone who chooses to do this that some lenders charge penalties (I've got a mortgage with CitiBank, against my will, and they do this) for payments made outside a certain time window in a month. Since I have mortgage payments draft directly from my bank account without writing a check, any additional (one-time) payment I make would be separate from my regular mortgage payment -- I would write a check. If that check gets to CitiBank after a certain day of the month (the 5th or 8th, I think) they'll charge me a service charge.

Of course, they've offered me the "opportunity" to make half-payments every two weeks -- resulting in 13 payments a year rather than 12 -- but the fee attached to that structure is ridiculous, too. They tell me I'll save in interest, but don't bother working in the cost of their fee to their calculations.