Leas: Little Extensible Accounting System
I wrote the “Little Extensible Accounting System” or Leas when I was budgeting for my wedding. It’s kind of the Emacs of personal finance (without the popularity). You can extend it by writing Scheme (a Lisp dialect) even though the core system is in C. Most of the commands are written in Scheme that calls C functions. I use the extensibility to do things like fetch the latest stock/mutual fund prices, etc.