DSBudget is a very simple budget management application. It is free and open source. It is a multi-platform tool, and works on Windows, Mac, and Linux. It does have lots of bells and whistles, but is a good tool if you want to just keep track of your incomes and expenses. It lets you add all the incomes, and all the expenses, and keep track of running total.
DSBudget also lets you create budgets for different expense categories. For example, you can create budgets for Groceries, Entertainment etc. When you add an expense, you can assign it to one of those expense categories, and DSBudget keeps track of if you are exceeding your budget. DSBudget has a very simple visual representation of the budget that you did for each category, and how much have you actually spent in that category.
As I mentioned in the beginning, DSBudget is not the top of the line tool in market for budgeting and financial management. But it definitely is one of the simplest. So, if you have never done financial planning or budgeting before, this might be a good financial planning software to start with. Another simple tool with more features is BudgetSketch. If you are into stocks, you need to read about JStock.