Older ntfs drivers on Lion and latter
I've tried open-sourced packages NTFS-FREE and FUSE for OS X on Mac OS X Mountain Lion v10.8.2 and can read/write NTFS partition successfully, although NTFS-FREE only claimed to support OS X up to v10.7.

You must install both of the following packages to get things work.

FUSE for OS X:
http://osxfuse.github.com/

NTFS-FREE:
http://sourceforge.net/projects/ntfsfree/files/