My most useful ones are honestly just spreadsheets. I made a "family debts" google sheet that we can input expenses into, and split amongst ourselves, and then

My most useful ones are honestly just spreadsheets. I made a "family debts" google sheet that we can input expenses into, and split amongst ourselves, and then use to optimally resolve debts between each other. It gets a ton of use, and allows us to just settle up every few months.