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.
In Mac OS X Lion and Mountain Lion, you'll instead have to install the OSXFuse driver, a replacement for Macfuse which is no longer developed.
However, even with OSXFuse, NTFS-3G does not work fully in Lion and Mountain Lion; 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.