Software for My Finances

I really want to become better at budgeting my money and tracking expenditures, and I'm looking for some software that will help me do that. It needs to support multiple accounts, give me reports about my spending, and be simple use. There are lots of products out there that fit this description, but here's a catch - I also want it to be a cross-platform application since I am always switching among my Mac, Windows, and Linux OS's. Also, (and this is just the computer guy in me talking) I want it to be free and open source. Does anyone know of a piece of software that fits this description? After some searching, I think I might need to develop my own program that fits this description. It could be a big project, though. One idea is to create a server application that I could access remotely via a secure web connection. That way, I could balance my checkbook from anywhere in the world.