Starting from Mac OSX 10.4.1, a read only driver for NTFS comes built-in with operating system, so NTFS drives can be easily mounted in read only mode in Mac OSX. However, read/write can also be enabled for NTFS drives by using the NTFS-3G driver coupled with FUSE. Here is what you require:
1. Download and install MacFUSE.
2. Download and install stable version of NTFS-3G driver Mac OSX DMG package.
3. Restart your system.
Staring with NTFS-3G version 1.1120, this is all you need. Now whenever you plug-in your external NTFS drive, it will be automatically mounted in read/write mode.
If you want to revert to Apple's NTFS driver, you can use a script that comes with NTFS-3G dmg package. You can then use 'mount' command in Terminal to manually enable read/write on a particular drive, the details can be found in this article.
As a side note, FUSE is a really great idea and ever since it has been ported to Mac OSX, a number of interesting application have been built based on it. Take for example, GrabFS, which is a screenshot file system!