PearBudget is actually modeled off of the envelope method. In fact, many people who use the envelope method use PearBudget alongside it, as a record of their expenses, so they can look back over time and see how their spending habits have changed. (Don’t you wonder sometimes how much you were spending on various categories a year or two ago? PearBudget will help you see that.)
Each of your categories acts as your “digital envelope” — when you have money in the category, you can spend it. When you don’t have money in the category, it’s time to stop spending.
Switching money from one “envelope” to another is easy. Let’s say you want to move $10.00 from the “clothing” category to the “groceries” category. All you’d do would be to enter a “receipt” of $10.00 in the clothing category and a “receipt” of -$10.00 (that’s negative $10.00) in the “groceries” category. The -$10.00 will even show up as green, to highlight that it’s a credit. You’d then have an extra $10.00 in groceries. (And, if you wanted to, you could tag the entries with something like “transfer from clothing to groceries” to help you recall the transfer down the road.)