If you have an android device, 'Karl's mortgage calculator' is fantastic. It may be available for i products, too.
You can modify all sorts of parameters and expenses, and add in prepayments monthly, semi-annually, annually, one time, etc. You can also choose the start date for prepayments, such as start paying extra $500 monthly on Jan 1, 2015. It gives nice graphs and tables in various forms to see impact on interest/principal, payoff date, etc.
In the settings menu, you can choose bi weekly and accelerated bi weekly payments.
It comes in free or 'pro' (I think pro is just ad-free). I use it a lot, so I paid the $2 or whatever to get the pro version.
