How do I write to NTFS drives in OS X?
Updated at May 2015, with current solutions.

Free solutions

FUSE for OS X - Successor to MacFUSE

NTFS-3G
Requires a build from source for anything newer than 2010. Last "pre built" version linked from http://macntfs-3g.blogspot.co.uk/2010/10/ntfs-3g-for-mac-os-x-2010102.html You can build it from source using homebrew in the other answers.

Paid solutions

Paragon NTFS

Tuxera NTFS



MacFuse is abandonware, and hasn't been updated since 2009.