WyCash

WyCash is a fixed-income portfolio management system. It is the first commercial software developed and maintained using all ExtremeProgramming principles and is therefore a noteworthy first instance of ExtremeProgramming Projects. WyCash is in use today generating revenue for vendor and customer alike a decade after its introduction. WyCash started as an ordinary Smalltalk Language project, with a prototype that evolved into a product. The WyCash team stayed with the product through many many releases. By continuously evolving their own practices the team created most of those of today's XP. https://wiki.c2.com/?WyCash

WyCash methods differed from XP in two subtle and probably unimportant ways ...

• Tasks were posted to a WorkQueue (Development Queue) as needed rather than at fixed intervals of XP. In common was the notion that management could reorder work, but not interfere with the estimated durations.
• A single testing framework served the needs of both developers and customers rather than the separate unit and functional tests of XP. In common was the recognition that quality regression tests were mandatory for aggressive refactoring. (TDD)

Ward Cunningham describes this as his favorite product/project ever.