I looked around quite a bit before I found this answer. I needed an NTFS drive to work in a pinch and this trick saved the day.
CNet has an article on how to mount and use an NTFS drive on OS X I've tested it in Mountain Lion and it worked for my purposes. Keep in mind, however, that support is not well tested. Be careful.
How to Manually Enable NTFS Read and Write
Here's a summary of the process.
Here's the line to add to the fstab file. Replace 'name' with the name of your drive.
LABEL=name none ntfs rw,auto,nobrowse
Note: If you have a space in the disk name, you can use the characters '\040' in place of that space in the /etc/fstab file. For example, my drive was called 'FreeAgent Kay' and I used the following line.
LABEL=FreeAgent\040Kay none ntfs rw,auto,nobrowse