I would recommend: http://www.mybudgetview.com. It is significantly cheaper than it's competitors with native support for windows, linux and mac as it's developed in java. It also has a intuitive and well designed UI and even can download your statement direct from your bank (if supported). Is much as i liked YNAB i found their pricing to be a little steep which is ironic to say what the application is intended for.
This is exactly the question I had a few years ago: how can one visualize the estimated evolution of one's budget in the next few months, to plan purchases or projects such as vacations, etc.? I then started building a new application with a friend, to manage just that. The key to our work has been to find a way to estimate the evolution of a personal budget over time. We now have many users in France, who use it to plan and monitor their budget and projects. We just published the english version of the tool, and the next step for us is to see if it fits user needs in the US and other countries, who may have different spending or budgeting habits. Maybe you would like to give a shot? Our application is called BudgetView, and you can find it here: http://www.mybudgetview.com We'll be happy to have your feedback, and see how we can taylor the tool to suit your needs. What do you think? Régis.