Personal Financier is a tool to track expenses by category and then project the impact to the overall budget. Underspending in a given month increases the budget available to use in future months, and overspending reduces it. In this way, the budget adjusts over time to reflect actual spending.
The easiest way to try it out is to use the demo account (password demo) that gets automatically setup on install. Enjoy!
(See Wiki for more detailed information)