LAST UPDATED: December 10, 2013 (Mavericks support)
NTFS-3G is a Mac app that enables reading & writing to NTFS drives on Mac OS X Snow Leopard for free. You need to install the Macfuse driver as well, for NTFS-3G to work. Once you've installed both NTFS-3G and Macfuse, you'll be able to write files to your Windows hard drives flawlessly.
Unfortunately, development of the Macfuse driver has stopped, so there will likely never be any official support for newer versions of Mac OS X. In Mac OS X Lion, Mountain Lion, and Mavericks, you'll instead have to install the OSXFuse driver, a replacement for Macfuse (be sure to checkmark the "MacFUSE Compatibility Layer" when running the OSXFuse installer).
However, even with OSXFuse, NTFS-3G does not work fully in Lion, Mountain Lion, or Mavericks; you will still get mounting errors every time your computer boots. To fix these errors, you'll have to install this patch for NTFS-3G.
These post-Snow Leopard compatibility issues affect all free NTFS enablers, including apps like iNTFS and NTFS Mounter.
Paragon NTFS ($20)
Paragon NTFS is a simple $20 app that enables writing to NTFS drives on Mac OS X, with no hassle involved.
This app and Tuxera NTFS claim to offer better reading/writing performance than the standard NTFS-3G + Macfuse/OSXFuse setup, though I haven't verified this myself. Tuxera costs $32 and essentially does the same thing as Paragon NTFS, which is why I recommend Paragon's app instead.
Though both NTFS-3G and Paragon NTFS work flawlessly for dragging and dropping files into your Windows hard drives from Finder, they're not perfect solutions. Your Windows hard drives have to be remounted every time you reboot, so they're not always available the moment Mac OS X starts up; you have to wait for Disk Utility to mount them. This can lead to issues with apps that try to access those drives at startup, e.g. Dropbox.
SEE ALSO: How to access your HFS+ hard drives on Windows