If you mean the original Xbox (old black beauty) then no, there is no way that I can think of file sharing on a network that does not involve modding the xbox.
If you mean the newer Xbox360, then it is possible to stream movies, music etc across the network from Windows / Mac PCs.
There are also devices that sit between the hard drive and the Xbox360 called an X-Sata, which allows you to connect the hard drive directly to a PC, but as far as I can tell this is only useful for moving videos, demos savegames etc off the hard drive for backup.
I liked the idea of being able to move files and so on on to my pc hard drive which has more room to store things like downloaded games and map updates on it than the xbox hard drive. I just don't like the sound of invalidating the warranty in the 360 by software hacks and mod chips!