The most widely recognized tool for general Forza modding on Xbox 360.
If you’re interested in the technical aspects (file structure, hashing, etc.) for educational or offline/archival purposes, let me know, and I can explain how save resigning works in general without promoting cheating.
If you want safe, legal alternatives, choose one:
Most free editors you find are "Lite" versions. They let you change money, but crash when you touch car IDs. A version typically includes: