EmuNations

xemu (fork of xqemu, which it was forked from QEMU) is an open-source, cross-platform, low-level system emulator for the original Xbox and Sega Chihiro. It's still in experimental state, which means it has partial audio support, graphics rending isn't consistant but it does have save state, gamepad, network support, etc. You'll also need an XBOX bios to run it. 

Cxbx-Reloaded (fork of Cxbx) is a work-in-progress Xbox emulator for Windows and WINE.  Thanks to recent developments, it boots most of the game library and continues to make steady progress. It contains it's own implementation of the Xbox kernel, which means there's no need to acquire a dump from your Xbox. It also has audio and gamepad support but very few gaming enhancements.