Recommended background readings are How Much House Can You Afford?; How Much House Should You Buy?;  Rent or Buy? A Simpler Approach: and Developing a Lease-to-Own Market.

Housing Affordability Calculator.
For borrowers trying to figure out whether they can afford a given-priced house.

Down Payment Calculator: Small Down Payment Now or Larger One Later.
For borrowers trying to decide whether they will be better off buying a home now with a small or no down payment, or saving for a larger down payment and buying later. This is often described as "rent versus buy".

Lease-to-Own Calculator,
For home seller/investors who expect to realize more from the sale of their home by offering an option to purchase to a hopeful buyer than from an immediate sale.