Interest-Only Mortgage

Whether your mortgage is interest-only, adjustable-rate or whatever -- every time you send in an extra dollar toward the principal, you don't owe interest on that dollar from them on.  If you had a 6% mortgage, that dollar would be saving you six cents every year your mortgage lasted.

It all depends how you want to invest that extra dollar -- for a 6%, no-risk, guaranteed return, or someplace else.  The only other consideration would be if your mortgage carried any pre-payment penalty.