I recently discovered this handy little open source tool from Erik Larsson.
As standard, Mac OS X offers the ability to read NTFS volumes, but not to write to them.
NTFS-3G looks to remedy this, and unlike other tools on the market, is free to download, although I would encourage you to donate to Erik using the Paypal link on his site if you find this useful.
This is a really handy tool if like us in support at root6, you find yourself flitting between platforms on a regular basis.
Grab NTFS-3G from here.