I've have had some experience with above mentioned LedgerSMB, but keeping track of data is what a computer is best at and you could probably just as easily script your own software (just as I keep some sh and TeX scripts around that generate my invoices). What you really need as a small business owner are codified accounting rules. Your software needs to know that this year there is a special tax break on people below age x so the value y should be calculated in a different way for them. Those are specific to both year and locale of course. When you're very small and want that done with software there's not much choice.