Ledger.

Which mode to use. Simple is what most people mean by a budget: decide what each category gets in a month, log what you spend, watch the remainder. Zero-based is stricter and more effective — you take the income that has actually arrived and assign every dollar of it to something, including savings, until nothing is left unassigned. The strength of the second is that money without a job tends to disappear. The cost is that it asks more of you. Switch between them freely; the same categories and spending feed both.

Why there's no bank connection. Linking accounts is the feature budgeting apps charge for, and it works by handing your banking credentials to a middleman that then holds a copy of every transaction you make. Entering things by hand takes a couple of minutes a week and keeps all of it on your own machine. It also makes you look at each purchase once, which is a bigger part of why budgeting works than any chart.