ReactOS 0.4.15 is the largest release to date, focusing on compatibility, bug fixes, and improved system stability. It introduces new features such as registry mechanisms, enhanced security, and improvements to system tools. The release supports 64-bit x86 architecture and includes significant updates to the Plug and Play Manager, audio enhancements, and better memory management. Users can now boot from USB devices and utilize more third-party drivers. The update also features quality-of-life improvements in tools like Notepad and Paint, and a new visual style. Future upgrades are promised, including power management and UEFI support. ReactOS 0.4.15 is available for download, but it remains in alpha status.