There's http://hackage.haskell.org/package/hledger-interest, a hledger addon.

Any spreadsheet software will do this. =money_in_account*(interest_rate/12)

I would advise to start with a spreadsheet and then move to custom accounting software after you are used to keeping track of numbers.

Hmm, I suppose spreadsheets would work, but I'd like if there was something with a friendlier interface for seeing how much someone owes me after a certain time, and with partial payments.

That formula won't work with compound interest. I'm not sure if most spreadsheet packages have an exponential function, but the formula should be

money_in_account * exp( interest_rate/12 )